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  • Equivalent Weight in PU Foam: Calculation Guide

    Equivalent Weight in PU Foam: Calculation Guide

    Introduction

    Equivalent weight is one of the most important calculation values in polyurethane foam formulation.

    It is also one of the most common sources of hidden formulation errors.

    A foam formula can look correct on paper. The index may appear correct. The raw material parts may look familiar. The production team may check catalysts, silicone, temperature, density, and machine settings. But if even one equivalent weight value is wrong, the entire stoichiometric balance can be wrong.

    This is why equivalent weight matters.

    Equivalent weight is the value that connects raw material data to polyurethane chemistry. It converts each reactive component into a common basis so the formulator can calculate isocyanate demand correctly.

    Polyol, isocyanate, water, and crosslinkers all have different structures and different reactive groups. Equivalent weight allows all of them to be compared on the same chemical basis.

    This guide explains what equivalent weight means, how it differs from molecular weight, and how to calculate equivalent weight for every major PU foam component.

    What Is Equivalent Weight?

    Equivalent weight answers one simple question:

    How many grams of this material contain one equivalent of reactive groups?

    In polyurethane formulation, equivalent weight is not just a theoretical value. It is the foundation of stoichiometric balance. It tells the formulator how much of a material is required to provide one mole-equivalent of reactive functionality.

    For example:

    • Polyol provides hydroxyl groups.
    • Isocyanate provides NCO groups.
    • Water provides reactive hydrogens.
    • Crosslinkers provide hydroxyl, amine, or other active hydrogen groups.

    Each of these materials has a different molecular weight and a different number of reactive groups. Equivalent weight normalizes them so they can be used in the same calculation system.

    Without equivalent weight, the isocyanate index calculation has no reliable foundation.

    Equivalent Weight vs Molecular Weight

    A common mistake is confusing equivalent weight with molecular weight. They are not always the same.

    • Molecular weight is the mass of one mole of complete molecules.
    • Equivalent weight is the mass that contains one mole-equivalent of reactive groups.

    For a monofunctional material, molecular weight and equivalent weight can be the same. But for materials with more than one reactive group, equivalent weight is lower than molecular weight.

    The general relationship is:

    Equivalent Weight = Molecular Weight ÷ Functionality

    For example, a trifunctional polyol with molecular weight 3,000 g/mol has three reactive hydroxyl groups per molecule.

    So:

    EW = 3,000 ÷ 3 = 1,000 g/eq

    This means 1,000 grams of that polyol contains one equivalent of hydroxyl reactivity.

    The same principle explains why water has an equivalent weight of 9, not 18. Water has a molecular weight of 18, but it has two reactive hydrogens involved in the isocyanate reaction.

    So:

    EW water = 18 ÷ 2 = 9 g/eq

    This distinction is critical. A formulation that uses molecular weight where equivalent weight is required can produce a completely wrong index calculation.

    Diagram explaining equivalent weight versus molecular weight in polyurethane formulation
    Molecular weight measures the whole molecule. Equivalent weight measures the mass per reactive group.

    Why Equivalent Weight Matters in PU Foam Formulation

    Polyurethane foam chemistry is based on the reaction between isocyanate groups and active hydrogen groups.

    The key reaction balance is:

    • NCO groups from isocyanate
    • OH groups from polyol
    • Reactive hydrogens from water
    • Reactive groups from crosslinkers or chain extenders

    The isocyanate index depends on these equivalent relationships.

    If the equivalent weight of one component is wrong, the calculated number of reactive equivalents is wrong. If the reactive equivalents are wrong, the isocyanate requirement is wrong. If the isocyanate requirement is wrong, the actual foam properties can shift.

    This can affect:

    • Foam hardness
    • Compression set
    • Resilience
    • Crosslink density
    • Cure behaviour
    • Aging stability
    • Batch-to-batch consistency

    Equivalent weight errors are dangerous because the foam may still rise and look normal. The problem usually appears later in physical testing or customer use.

    How to Calculate Polyol Equivalent Weight

    For polyols, equivalent weight is calculated from hydroxyl value.

    The formula is:

    Polyol EW = 56,100 ÷ OHV

    Where:

    • EW = equivalent weight in g/eq
    • OHV = hydroxyl value in mg KOH/g
    • 56,100 = conversion constant from the KOH titration basis

    The constant 56,100 comes from the molecular weight of potassium hydroxide (56.1 g/mol) multiplied by 1,000 for unit conversion.

    Example

    If a polyol has an OHV of 51 mg KOH/g:

    EW = 56,100 ÷ 51 = 1,100 g/eq

    So a polyol with OHV 51 has an equivalent weight of approximately 1,100 g/eq. This means 1,100 grams of that polyol contains one equivalent of reactive hydroxyl groups.

    This calculation should be done using the actual OHV from the Certificate of Analysis, not only the nominal value from the Technical Data Sheet.

    Polyol equivalent weight formula using hydroxyl value in polyurethane foam formulation
    Polyol equivalent weight is calculated from hydroxyl value using EW = 56,100 ÷ OHV.

    How to Calculate Isocyanate Equivalent Weight

    For isocyanates, equivalent weight is calculated from the percentage of NCO.

    The formula is:

    Isocyanate EW = 4,200 ÷ %NCO

    Where:

    • EW = equivalent weight in g/eq
    • %NCO = actual NCO percentage from the Certificate of Analysis
    • 4,200 = molecular weight of the NCO group (42 g/mol) multiplied by 100

    Example 1: TDI 80/20

    If TDI has a %NCO of 48.3:

    EW = 4,200 ÷ 48.3 = 86.96 g/eq

    So the TDI equivalent weight is approximately 87 g/eq.

    Example 2: MDI

    If MDI has a %NCO of 31.5:

    EW = 4,200 ÷ 31.5 = 133.33 g/eq

    So the MDI equivalent weight is approximately 133 g/eq.

    The same formula applies to TDI, MDI, polymeric MDI, and modified isocyanates. The constant does not change. The variable is the actual %NCO value.

    For production calculation, use the %NCO from the Certificate of Analysis, not only the general TDS range.

    Isocyanate equivalent weight formula using percent NCO for TDI and MDI
    Isocyanate equivalent weight is calculated from actual %NCO using EW = 4,200 ÷ %NCO.

    How to Calculate Water Equivalent Weight

    Water is one of the most important components in flexible polyurethane foam formulation. It is also one of the easiest to calculate incorrectly.

    Water has a molecular weight of 18 g/mol. But its equivalent weight in polyurethane formulation is not 18.

    Water has two reactive hydrogens involved in the isocyanate reaction sequence. One water molecule consumes two NCO groups.

    Therefore:

    Water EW = 18 ÷ 2 = 9 g/eq

    This value is fixed.

    For PU foam index calculation: water equivalent weight is 9, not 18.

    Using 18 instead of 9 cuts the calculated water contribution in half and can severely distort the isocyanate index calculation.

    The detailed water equivalent weight error and its production consequences are covered in a separate article — the water EW mistake is one of the most damaging single-number errors in PU foam formulation.

    Water equivalent weight is 9 not 18 in polyurethane foam formulation
    Water has two reactive hydrogens, so its equivalent weight in polyurethane formulation is 9 g/eq.

    How to Calculate Crosslinker Equivalent Weight

    Crosslinkers and chain extenders must also be included in equivalent weight calculations if they contain reactive groups.

    For hydroxyl-based crosslinkers, the same formula used for polyols can often be applied:

    Crosslinker EW = 56,100 ÷ OHV

    Example: Glycerol

    If glycerol has an OHV of approximately 1,827 mg KOH/g:

    EW = 56,100 ÷ 1,827 = 30.7 g/eq

    So the equivalent weight is approximately 31 g/eq.

    This is much lower than the equivalent weight of a typical flexible foam polyol. That means even small quantities of crosslinker can contribute meaningful reactive equivalents.

    Important note about amine-functional crosslinkers

    Some crosslinkers or chain extenders contain more than hydroxyl groups. For example, some amine-functional materials include reactive amine hydrogens as well. In those cases, an OHV-only calculation may not capture all reactive functionality.

    The correct approach is to account for all active hydrogen groups that react with isocyanate.

    This topic is covered in more depth in a separate article on equivalent weight mistakes, because missing reactive groups in crosslinkers can quietly distort index and network structure.

    Crosslinker equivalent weight calculation using hydroxyl value in polyurethane foam formulation
    Hydroxyl-based crosslinkers use the same EW formula as polyols, but their low EW can strongly affect reactive balance.

    Complete Equivalent Weight Reference Table

    The table below summarizes the main equivalent weight formulas used in PU foam formulation.

    ComponentEW FormulaKey VariableWorked Example
    Polyol56,100 ÷ OHVOHV from CoAOHV 51 → EW 1,100
    Isocyanate4,200 ÷ %NCO%NCO from CoA48.3% NCO → EW 86.96
    Water18 ÷ 2Fixed valueEW = 9
    Hydroxyl crosslinker56,100 ÷ OHVOHV of crosslinkerOHV 1,827 → EW 30.7

    Every number in this table can feed into the isocyanate index calculation.

    If one EW value is wrong, the index becomes unreliable. If multiple EW values are wrong, the production symptoms can become confusing and difficult to diagnose.

    How Equivalent Weight Feeds Into Isocyanate Index

    Equivalent weight is used to calculate the number of reactive equivalents in the formula.

    The general formula is:

    Reactive Equivalents = Parts by Weight ÷ Equivalent Weight

    For example, if a formulation contains 100 parts of polyol with EW 1,100:

    Polyol equivalents = 100 ÷ 1,100 = 0.09091

    If the formula contains 4 parts of water with EW 9:

    Water equivalents = 4 ÷ 9 = 0.44444

    Each reactive component is converted into equivalents. Then all reactive hydrogen equivalents are added together. The isocyanate required is calculated from that total and the target index.

    This is why equivalent weight is not an isolated calculation. It is part of the full stoichiometric system.

    Wrong EW → wrong equivalents → wrong index → wrong foam properties.

    Workflow showing equivalent weight calculation feeding into isocyanate index calculation in PU foam formulation
    Equivalent weight is the first step in calculating reactive equivalents and isocyanate index

    Practical Rules for Equivalent Weight Calculation

    Use these rules to avoid common formulation mistakes:

    1. Do not confuse molecular weight with equivalent weight. Molecular weight describes the whole molecule. Equivalent weight describes the mass per reactive group.
    2. Use actual CoA values when available. Polyol OHV and isocyanate %NCO can vary by batch.
    3. Use water EW = 9. Water has two reactive hydrogens and consumes two NCO groups.
    4. Recalculate EW when OHV changes. Polyol equivalent weight is not fixed if OHV changes.
    5. Recalculate isocyanate EW when %NCO changes. The isocyanate equivalent weight depends on actual %NCO.
    6. Include crosslinkers and chain extenders. Any reactive component must be included in the stoichiometric calculation.
    7. Check all active hydrogens. Some materials contain amine groups or other reactive functionality not captured by simple OHV alone.
    8. Audit old formula sheets. Legacy spreadsheets often contain copied EW values that may no longer match current raw material data.

    Use the PolymerIQ Equivalent Weight Calculator

    Manual calculation is useful because every foam engineer should understand the chemistry behind equivalent weight. But in production, the calculation must also be fast and consistent.

    The PolymersIQ Equivalent Weight Calculator helps you calculate equivalent weight from OHV quickly.

    Use it when:

    • A new polyol batch arrives
    • The CoA OHV is different from the design value
    • You are checking a formulation before production
    • You are preparing an isocyanate index calculation
    • You are auditing an old formula sheet

    Open the Equivalent Weight Calculator →

    For a deeper article on the water calculation error, read Why the Equivalent Weight of Water Is 9 in Polyurethane Foam.

    For common production mistakes, read 5 Equivalent Weight Mistakes That Damage PU Foam Production.

    For the full isocyanate index method, read Isocyanate Index Calculation Guide for PU Foam Engineers.

    FAQs

    What is equivalent weight in polyurethane foam formulation?

    Equivalent weight is the mass of material that contains one mole-equivalent of reactive groups. In polyurethane foam, it is used to convert each reactive component (polyol, isocyanate, water, crosslinker) into a common basis so the formulator can calculate isocyanate demand and index correctly.

    How is equivalent weight different from molecular weight?

    Molecular weight is the mass of one mole of complete molecules. Equivalent weight is the mass per reactive group. For monofunctional materials they can be the same, but for multifunctional materials, equivalent weight is lower than molecular weight. The relationship is EW = Molecular Weight ÷ Functionality.

    How do I calculate polyol equivalent weight?

    Use EW = 56,100 ÷ OHV, where OHV is the hydroxyl value in mg KOH/g. The constant 56,100 comes from the molecular weight of potassium hydroxide (56.1 g/mol) multiplied by 1,000 for unit conversion. Always use the actual OHV from the Certificate of Analysis, not the nominal TDS value.

    How do I calculate isocyanate equivalent weight?

    Use EW = 4,200 ÷ %NCO, where %NCO is the percentage of NCO groups by weight. The constant 4,200 comes from the NCO group molecular weight (42 g/mol) multiplied by 100. The same formula applies to TDI, MDI, polymeric MDI, and modified isocyanates — only the %NCO value changes.

    Why is the equivalent weight of water 9 and not 18?

    Water has a molecular weight of 18, but each water molecule has two reactive hydrogens and consumes two NCO groups during the blowing reaction. So the equivalent weight is 18 ÷ 2 = 9 g/eq. Using 18 instead of 9 cuts the calculated water contribution in half and severely distorts the isocyanate index.

    Do I need to calculate equivalent weight for crosslinkers?

    Yes. Hydroxyl-based crosslinkers use the same formula as polyols (EW = 56,100 ÷ OHV). Glycerol, for example, has an OHV around 1,827 mg KOH/g, giving an EW of about 31 g/eq. Because crosslinker EW is much lower than polyol EW, even small amounts contribute meaningful reactive equivalents to the calculation.

    What about amine-functional crosslinkers and chain extenders?

    Materials with amine groups or other active hydrogens cannot be captured by an OHV-only calculation. The correct approach is to account for all active hydrogen groups that react with isocyanate. Missing reactive groups in crosslinkers can silently distort the index and the polymer network.

    How does equivalent weight feed into the isocyanate index?

    Reactive equivalents are calculated as Parts ÷ Equivalent Weight for each component. All reactive hydrogen equivalents are summed, then multiplied by the target index to determine required NCO equivalents. The isocyanate quantity is then calculated as Required NCO equivalents × Isocyanate EW. Wrong EW values create wrong equivalents and wrong index.

    Should I recalculate equivalent weight when raw material batches change?

    Yes. Polyol EW changes when OHV changes. Isocyanate EW changes when %NCO changes. Treating EW as a fixed value copied from an old formula sheet is one of the most common causes of hidden formulation drift.

    What’s the most common equivalent weight mistake in PU foam formulation?

    Using water EW as 18 instead of 9. Because water is usually one of the largest contributors to reactive hydrogen equivalents in flexible foam, getting this single value wrong can shift the running index by many points and produce foam that is significantly harder than expected.

    Key Takeaways

    Equivalent weight is the mass of material that contains one equivalent of reactive groups. It is not always the same as molecular weight.

    In polyurethane foam formulation, equivalent weight is needed for every reactive component because the isocyanate index depends on reactive equivalents.

    The main formulas are:

    • Polyol EW = 56,100 ÷ OHV
    • Isocyanate EW = 4,200 ÷ %NCO
    • Water EW = 18 ÷ 2 = 9
    • Hydroxyl crosslinker EW = 56,100 ÷ OHV

    Equivalent weight should be treated as a live calculation, not a fixed value copied from an old formula sheet.

    • If OHV changes, polyol EW changes.
    • If %NCO changes, isocyanate EW changes.
    • If water is entered as 18 instead of 9, the index calculation becomes seriously wrong.

    A correct equivalent weight system is the foundation of a correct isocyanate index calculation.

    Conclusion

    If your foam formula has been adjusted many times over the years, the equivalent weight values in the spreadsheet may no longer be correct.

    PolymersIQ can help review your formulation, check every equivalent weight value, and identify whether hidden stoichiometric errors are affecting foam quality.

    To get accurate support, please share:

    • Polyol grade, OHV, and supplier
    • Isocyanate type and %NCO from the Certificate of Analysis
    • Water level and any crosslinkers or chain extenders in use
    • Current EW values used in the formula sheet
    • Description of the foam quality issue (if any)

    Contact PolymerIQ for a stoichiometric formulation audit →

  • Hydroxyl Value in Polyurethane Foam: What OHV Means and How to Calculate Equivalent Weight

    Hydroxyl Value in Polyurethane Foam: What OHV Means and How to Calculate Equivalent Weight

    Introduction

    Most polyurethane foam quality problems are blamed on the machine.

    The machine is not always the problem.

    In many foam plants, the real cause is sitting inside the raw material data — especially the hydroxyl value of the incoming polyol batch.

    A formula may be developed using one polyol OHV value, but the next delivery may arrive with a slightly different OHV. The value may still be inside the supplier’s specification range. It may pass incoming QC. It may not trigger any alarm.

    But if nobody recalculates the equivalent weight, the formulation is no longer running at the same chemical balance. The formula looks the same on paper, but it behaves differently in production.

    This is why hydroxyl value is one of the most important numbers in polyurethane foam formulation. It controls the equivalent weight of the polyol, affects isocyanate demand, and directly influences the final foam properties.

    This article explains what hydroxyl value means, how it relates to equivalent weight, and how to calculate it correctly for PU foam production.

    What Is Hydroxyl Value?

    Hydroxyl value, often written as OHV, measures how many reactive hydroxyl groups are present in one gram of polyol.

    It is expressed as:

    mg KOH/g

    This means milligrams of potassium hydroxide equivalent per gram of sample.

    The potassium hydroxide is not actually inside the polyol. It is part of the measurement convention used in titration chemistry. The value gives formulators a standard way to compare the hydroxyl content of different polyols.

    In practical terms:

    • Higher OHV means more reactive hydroxyl sites per gram.
    • Lower OHV means fewer reactive hydroxyl sites per gram.
    • Higher OHV usually means shorter polyol chains.
    • Lower OHV usually means longer polyol chains.
    • Higher OHV generally produces stiffer foam behaviour.
    • Lower OHV generally produces softer, more flexible behaviour.

    This is why OHV is not just a laboratory number — it is a formulation control value.

    If OHV changes, the polyol equivalent weight changes. If equivalent weight changes, the isocyanate requirement changes. If the isocyanate requirement changes but the formulation is not recalculated, foam properties can shift.

    Diagram explaining hydroxyl value as reactive hydroxyl groups per gram of polyol
    Hydroxyl value represents the concentration of reactive OH groups in the polyol.

    Typical OHV Ranges for Different Foam Types

    Different polyurethane foam systems use polyols with very different hydroxyl value ranges.

    A flexible slabstock foam polyol is not the same as a rigid insulation foam polyol. The OHV range reflects the type of polymer network the formulation is designed to create.

    Foam TypeTypical OHV Range
    HR flexible foam28–35 mg KOH/g
    Flexible slabstock foam45–56 mg KOH/g
    Semi-rigid foam100–200 mg KOH/g
    Rigid / insulation foam350–550 mg KOH/g

    Flexible foams usually use lower-OHV polyols because they need longer, more elastic polymer chains.

    Rigid foams use much higher-OHV polyols because they require a dense, highly crosslinked structure.

    This is why OHV immediately tells you something about the intended application of a polyol. A polyol with OHV around 50 belongs to a very different formulation world than a polyol with OHV around 450.

    Typical hydroxyl value ranges for flexible foam, HR foam, semi-rigid foam, and rigid foam
    Different PU foam systems use different OHV ranges depending on flexibility, stiffness, and crosslink density.

    How Hydroxyl Value Is Measured

    Hydroxyl value is commonly measured using standard titration methods such as ASTM D4274 or ISO 14900. These are acetylation-based titration methods used to determine hydroxyl content in polyols.

    In production, the OHV value usually appears on the supplier’s Certificate of Analysis. For serious formulation control, the incoming CoA value should not be ignored or treated as a fixed number.

    The OHV value from each batch matters because every batch can have a slightly different hydroxyl value. Even if the value remains inside the supplier’s TDS specification range, it can still change the formulation balance.

    OHV and Equivalent Weight: The Critical Link

    Equivalent weight is the bridge between hydroxyl value and isocyanate stoichiometry.

    The formula is:

    Equivalent Weight = 56,100 ÷ OHV

    Where:

    • Equivalent weight is expressed in g/eq
    • OHV is expressed in mg KOH/g
    • 56,100 is the conversion constant based on potassium hydroxide molecular weight

    Equivalent weight tells you how many grams of polyol contain one equivalent of reactive hydroxyl groups.

    This value is essential because polyurethane formulation is based on equivalent relationships, not simply weight relationships.

    • A polyol with a lower OHV has a higher equivalent weight.
    • A polyol with a higher OHV has a lower equivalent weight.

    That matters because isocyanate demand is calculated from reactive equivalents.

    [IMAGE 4 — OHV TO EQUIVALENT WEIGHT FORMULA] Placement: After the section “OHV and Equivalent Weight”, before “Worked Example”. Filename: ohv-equivalent-weight-formula-polyurethane.jpg ALT text: Hydroxyl value to equivalent weight formula for polyurethane polyol calculation Caption: Equivalent weight is calculated from hydroxyl value using the formula EW = 56,100 ÷ OHV. ChatGPT image prompt: “Create a clean technical formula infographic on a white background showing the relationship between hydroxyl value and equivalent weight in polyurethane formulation. Display the formula: Equivalent Weight = 56,100 / OHV. Add simple labels: OHV in mg KOH/g, EW in g/eq, used for isocyanate stoichiometry. Include a polyol drum icon, calculator icon, and small OH group symbols. Professional engineering style, blue and grey color palette, clean and readable. No logos. No brand names.”

    Hydroxyl value to equivalent weight formula for polyurethane polyol calculation
    Equivalent weight is calculated from hydroxyl value using the formula EW = 56,100 ÷ OHV

    Worked Example: Calculating Polyol Equivalent Weight

    Let’s calculate equivalent weight using a polyol OHV of 51 mg KOH/g.

    Formula: EW = 56,100 ÷ OHV

    Calculation: EW = 56,100 ÷ 51 = 1,100 g/eq

    So a polyol with OHV 51 has an equivalent weight of approximately 1,100 g/eq. This means 1,100 grams of that polyol contains one equivalent of reactive hydroxyl groups.

    Now compare that to different OHV values within a typical flexible foam range:

    OHV (mg KOH/g)Equivalent Weight (g/eq)
    451,247
    471,194
    511,100
    531,058
    551,020

    This table shows why OHV cannot be ignored.

    A shift from OHV 45 to OHV 55 creates an equivalent weight change of more than 200 g/eq.

    That is a large stoichiometric difference, even though the polyol may still be inside a normal supplier specification range.

    Table-style infographic showing hydroxyl value changes and equivalent weight shift in PU foam polyol
    Small OHV changes can create large equivalent weight shifts, affecting the formulation balance.

    Why OHV Changes Foam Behaviour

    OHV affects foam behaviour because it changes the number of reactive sites available in the polyol.

    If OHV is higher:

    • There are more reactive sites per gram.
    • Equivalent weight is lower.
    • Isocyanate demand increases.
    • The foam may trend softer if isocyanate is not adjusted correctly.
    • The final network balance may shift.

    If OHV is lower:

    • There are fewer reactive sites per gram.
    • Equivalent weight is higher.
    • Isocyanate demand decreases.
    • If isocyanate is not adjusted, the actual index can rise.
    • The foam may become harder than expected.

    This is one of the most important diagnostic relationships in PU foam formulation:

    OHV low → equivalent weight high → actual index can increase → foam can become harder

    OHV high → equivalent weight low → actual index can decrease → foam can become softer

    This does not mean OHV is the only factor controlling hardness. Catalyst, water, silicone, crosslinker, density, temperature, and machine delivery also matter.

    But if hardness changes batch to batch and the formulation looks unchanged, OHV should be checked early.

    Diagram showing how low OHV can increase index and hardness while high OHV can lower index and soften foam
    The direction of OHV drift helps diagnose whether foam may trend harder or softer.

    Why TDS OHV Is Not Enough

    A polyol Technical Data Sheet gives a specification range.

    That range tells you what the supplier considers acceptable for the product grade. It does not tell you the actual OHV of the batch sitting in your plant today.

    For example, a TDS may show:

    OHV range: 45–55 mg KOH/g

    If the engineer uses the midpoint forever, the calculation may be wrong when the actual delivered batch is 47 or 55.

    The Certificate of Analysis gives the batch-specific OHV value. That is the number that should be used for equivalent weight calculation.

    The difference matters because equivalent weight is calculated directly from OHV. Using the wrong OHV means using the wrong equivalent weight. Using the wrong equivalent weight means the isocyanate requirement may not match the actual reactive demand.

    Infographic comparing TDS hydroxyl value range with Certificate of Analysis actual OHV value
    The TDS gives the allowed OHV range, but the CoA gives the actual batch value needed for calculation.

    Practical Calculation Workflow for Foam Plants

    A simple OHV workflow can prevent many formulation errors.

    Use this process for every incoming polyol batch:

    1. Receive the polyol Certificate of Analysis.
    2. Record the actual batch OHV.
    3. Calculate equivalent weight using EW = 56,100 ÷ OHV.
    4. Compare the new EW with your design value.
    5. Recalculate the isocyanate index if the difference is meaningful.
    6. Adjust the formula if required before production.
    7. Keep a batch-by-batch OHV log for each supplier and grade.

    This workflow is simple, but it prevents one of the most common sources of hidden formulation drift.

    The most important point is this: equivalent weight is not a one-time value. It changes when OHV changes.

    Use the PolymerIQ Equivalent Weight Calculator

    Manual calculation is useful because every foam engineer should understand the relationship between OHV and equivalent weight.

    But in production, the calculation must be fast, repeatable, and error-free.

    The PolymerIQ Equivalent Weight Calculator helps you convert OHV into equivalent weight instantly.

    Use it when:

    • A new polyol batch arrives
    • The CoA OHV differs from your design value
    • A formulation is being checked before production
    • Foam hardness changes without a clear process reason
    • You are preparing an isocyanate index calculation

    Open the Equivalent Weight Calculator →

    Hydroxyl value and equivalent weight are directly connected to isocyanate index. After calculating equivalent weight, the next step is to use it in the index calculation. For the full index calculation method, read Isocyanate Index Calculation Guide for PU Foam Engineers.

    FAQs

    What is hydroxyl value in polyurethane foam?

    Hydroxyl value (OHV) measures how many reactive hydroxyl groups are present in one gram of polyol. It is expressed in mg KOH/g (milligrams of potassium hydroxide equivalent per gram of sample). The KOH is not actually in the polyol — it is part of the titration measurement convention. OHV is a key formulation control value because it determines polyol equivalent weight and isocyanate demand.

    How is hydroxyl value measured?

    OHV is commonly measured using standard titration methods such as ASTM D4274 or ISO 14900, which are acetylation-based titration techniques. The value is reported on the supplier’s Certificate of Analysis for each batch.

    What is the difference between OHV and equivalent weight?

    OHV expresses hydroxyl content in mg KOH/g. Equivalent weight expresses how many grams of polyol contain one equivalent of reactive hydroxyl groups (g/eq). They describe the same chemistry but in different units. The conversion is EW = 56,100 ÷ OHV.

    Why is the equivalent weight formula 56,100 ÷ OHV?

    The constant 56,100 comes from the molecular weight of potassium hydroxide (56.1 g/mol) multiplied by 1,000 for unit conversion. Since OHV is reported in mg KOH/g, dividing 56,100 by OHV gives the grams of polyol that contain one equivalent of OH groups.

    What is the typical OHV range for flexible foam polyols?

    Standard flexible slabstock foam polyols typically have OHV in the range of 45–56 mg KOH/g. HR flexible foam polyols are usually 28–35 mg KOH/g. Semi-rigid foam polyols sit at 100–200 mg KOH/g, and rigid insulation foam polyols are much higher at 350–550 mg KOH/g.

    Should I use OHV from the TDS or the Certificate of Analysis?

    Always use the actual OHV from the Certificate of Analysis for the specific batch in production. The TDS gives a specification range, and using the midpoint can introduce calculation error when the actual batch sits at the edge of the range. Equivalent weight is calculated directly from OHV, so a wrong OHV means a wrong EW.

    How does OHV affect foam hardness?

    OHV affects hardness indirectly through equivalent weight and isocyanate stoichiometry. If OHV is lower than design (and isocyanate is not adjusted), the actual running index can rise and foam may become harder. If OHV is higher than design, the actual index can drop and foam may become softer. This is why OHV should be checked early when batch-to-batch hardness variation appears.

    What happens if I don’t recalculate equivalent weight when polyol batch changes?

    The formula sheet will look correct, but the real reactive equivalents in the system will be different from what the calculation assumes. The isocyanate amount may no longer match the actual reactive demand, and the running index will drift away from the target. This can cause hidden hardness, compression set, or recovery problems that are hard to trace.

    Can OHV variation cause batch-to-batch foam quality problems?

    Yes. Even when OHV stays inside the supplier’s TDS range, batch-to-batch variation can shift the equivalent weight by tens or hundreds of g/eq. If the formula is not recalculated for each batch, the actual running index changes silently and foam properties can drift between deliveries.

    How does OHV differ between flexible and rigid foam polyols?

    Flexible foam polyols have low OHV (typically 28–56 mg KOH/g), which gives long, elastic polymer chains and a flexible network. Rigid foam polyols have high OHV (typically 350–550 mg KOH/g), which produces a dense, highly crosslinked network with stiff structural properties. The OHV range tells you immediately what kind of foam the polyol is designed for.

    Key Takeaways

    • Hydroxyl value (OHV) measures the concentration of reactive hydroxyl groups in a polyol.
    • Higher OHV means more reactive sites per gram and lower equivalent weight.
    • Lower OHV means fewer reactive sites per gram and higher equivalent weight.
    • The equivalent weight formula is EW = 56,100 ÷ OHV.
    • A change in OHV changes equivalent weight. A change in equivalent weight changes the isocyanate demand. If the formula is not recalculated, the actual running index can shift.
    • The TDS range should not be used as a fixed formulation value. The batch-specific OHV from the Certificate of Analysis should be used for production calculation.
    • For consistent PU foam production, every incoming polyol batch should have its OHV recorded, equivalent weight recalculated, and formulation impact reviewed before production.

    Conclusion

    If your foam hardness is changing from batch to batch and the machine settings look stable, the incoming polyol OHV may be one of the first values to check.

    PolymersIQ can help review your formulation, calculate equivalent weight correctly, and identify whether raw material variation is affecting your production baseline.

    To get accurate support, please share:

    • Polyol grade and supplier
    • Current OHV from the Certificate of Analysis
    • Design OHV used in your original formulation
    • Water level, crosslinker, and any other reactive components
    • Isocyanate type and %NCO
    • Description of the quality issue you are facing

    Contact PolymerIQ for a formulation audit →