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  • Why Water Equivalent Weight Is 9 in Polyurethane Foam

    Why Water Equivalent Weight Is 9 in Polyurethane Foam

    Introduction

    The equivalent weight of water in polyurethane foam is 9, not 18.

    This is one of the most important rules in PU foam formulation — and one of the most damaging mistakes when entered incorrectly.

    Water has a molecular weight of 18 g/mol. Because of that, many engineers assume the equivalent weight of water is also 18. That assumption is wrong in polyurethane chemistry.

    In PU foam, one water molecule ultimately consumes two NCO groups through the blowing reaction sequence. That is why the equivalent weight is calculated as:

    Water EW = 18 ÷ 2 = 9 g/eq

    If a formula spreadsheet uses 18 instead of 9, the water contribution is cut in half. The total reactive hydrogen equivalents become wrong. The calculated isocyanate demand becomes wrong. The index shown on the formula sheet no longer matches the chemistry in the reactor.

    This article explains why water EW is 9, how the water-isocyanate reaction works, what happens when 18 is used by mistake, and how this error appears in foam production.

    Why Water Equivalent Weight Is Not 18

    Water has a molecular weight of 18 g/mol. But equivalent weight is not always the same as molecular weight.

    Equivalent weight means the mass of material that contains one equivalent of reactive functionality.

    In polyurethane foam, water has two reactive hydrogens involved in the isocyanate reaction sequence. That means one mole of water provides two equivalents of reactivity toward NCO.

    So the calculation is:

    Water EW = Molecular Weight ÷ Reactive Hydrogen Count = 18 ÷ 2 = 9 g/eq

    For polyurethane foam index calculation, the correct value is Water EW = 9, not 18.

    Using 18 treats water as if it had only one reactive hydrogen. That cuts the water contribution in half and corrupts the index calculation.

    Water molecular weight 18 versus equivalent weight 9 in polyurethane formulation
    Water molecular weight is 18, but its equivalent weight in PU foam is 9 because it provides two reactive equivalents.

    How Water Reacts with Isocyanate in PU Foam

    Water reacts with isocyanate in two main stages.

    Stage 1: Water reacts with NCO. Water reacts with an isocyanate group to form an unstable carbamic acid intermediate. This intermediate quickly decomposes, releasing carbon dioxide (which blows the foam and forms cells) and producing a primary amine.

    Stage 2: The amine reacts with another NCO group. The amine formed in Stage 1 is reactive. It reacts with a second isocyanate group to form a urea linkage.

    This means one water molecule ultimately consumes two NCO groups:

    • One NCO in the initial water reaction
    • One NCO in the amine-to-urea reaction

    This is the chemical reason water equivalent weight is 9. It is not an approximation or a rule of thumb — it comes directly from the reaction mechanism.

    Water reaction with isocyanate showing two NCO groups consumed in polyurethane foam
    One water molecule reacts through a sequence that ultimately consumes two NCO groups.

    The Correct Water EW Calculation

    The calculation is simple:

    • Water molecular weight = 18 g/mol
    • Reactive hydrogens = 2
    • Water EW = 18 ÷ 2 = 9 g/eq

    This means 9 grams of water contain one equivalent of reactive hydrogen functionality for the PU foam index calculation.

    When calculating water equivalents in a formulation:

    Water Equivalents = Water Parts ÷ 9

    For example, if a flexible foam formula contains 4.0 parts water:

    4.0 ÷ 9 = 0.44444 equivalents

    If the formula uses EW = 18 instead:

    4.0 ÷ 18 = 0.22222 equivalents

    That is exactly half the correct value. The formula spreadsheet now believes there is much less reactive hydrogen demand than the chemistry actually has.

    Correct and wrong water equivalent weight calculation in PU foam formula
    Using EW = 18 cuts the calculated water equivalents in half compared with the correct EW = 9.

    Worked Example: How EW Water = 18 Corrupts the Index

    Let’s see how this mistake changes the full formula calculation.

    Example flexible slabstock formula:

    ComponentPartsCorrect EWCorrect Equiv.Wrong EWWrong Equiv.
    Polyol1001,1000.090911,1000.09091
    Water4.090.44444180.22222
    DEOA0.5310.01613310.01613
    Total H equiv.0.551480.32926

    The correct total reactive hydrogen equivalents are 0.55148. Using water EW = 18 gives 0.32926.

    Now assume the engineer targets Index 105 using the wrong equivalent system. NCO equivalents calculated from the wrong system:

    0.32926 × 1.05 = 0.34572

    But the actual correct reactive hydrogen equivalents are 0.55148. So the real running index is:

    0.34572 ÷ 0.55148 × 100 = 62.7

    The formula sheet says Index 105. The chemistry is running at approximately Index 62.7.

    This is not a small error. It is a completely wrong stoichiometric foundation.

    Water EW 18 causing wrong isocyanate index calculation in polyurethane foam
    Using water EW = 18 can make the formula sheet show Index 105 while the actual chemistry runs much lower.

    What This Error Looks Like in Production

    A water equivalent weight error does not always create a dramatic visual failure. The foam may still rise. The block may still form. Operators may not immediately see the problem at the machine.

    But the properties can be seriously wrong.

    If water is entered as 18 instead of 9 and the isocyanate quantity is calculated from that wrong value, the foam can be severely under-indexed.

    Common symptoms include:

    • Softer foam than expected
    • ILD below target
    • Poor compression set
    • Weak recovery
    • Slower or weaker cure
    • Tacky feel during early cure
    • Poor aging performance
    • Customer complaints after use
    • Confusing response to catalyst adjustments

    This kind of problem can be difficult to diagnose because it looks like a process issue. The team may adjust catalyst, silicone, cure temperature, water level, or crosslinker dosage. Some changes may improve one symptom temporarily. But the root cause remains inside the calculation.

    The spreadsheet must be checked.

    Production symptoms from wrong water equivalent weight causing under-indexed PU foam
    Wrong water EW can appear as soft foam, poor compression set, weak recovery, and confusing process variation.

    Why This Mistake Stays Hidden

    The water EW mistake stays hidden because the formula sheet often looks internally consistent.

    The numbers may be formatted correctly. The index cell may show the target value. The spreadsheet may have been used for years.

    But the spreadsheet is only as accurate as the assumptions inside it. If water EW is entered as 18, every downstream calculation built on that value becomes wrong.

    This mistake is especially common in legacy formulas because:

    • Water molecular weight is commonly remembered as 18
    • Engineers may copy old spreadsheets without checking the chemistry
    • The formula may have been empirically adjusted over time
    • Production teams may trust a formula because it has been used for years
    • Troubleshooting often focuses on machine and process variables first
    • The equivalent weight layer is rarely audited

    This is why a formula can carry the same error for months or years. The plant may keep adding practical corrections on top of a wrong calculation foundation. That creates a formula that works only by accident — and becomes difficult to transfer, scale, or troubleshoot.

    How to Check Your Formula Today

    Checking for this mistake is simple.

    Open your formula sheet and find the equivalent weight value used for water. It should be 9, not 18.

    Then check how the water equivalents are calculated:

    • Correct: Water equivalents = Water parts ÷ 9 (e.g., 4.0 ÷ 9 = 0.44444)
    • Wrong: Water equivalents = Water parts ÷ 18 (e.g., 4.0 ÷ 18 = 0.22222)

    After correcting the water equivalent weight, the full index must be recalculated. Do not change only the water EW cell and assume the formula is now production-ready. The isocyanate quantity may also need to be recalculated based on the correct total reactive hydrogen equivalents and target index.

    A safe review should include:

    1. Confirm water EW = 9
    2. Confirm polyol EW from actual OHV
    3. Confirm isocyanate EW from actual %NCO
    4. Confirm all crosslinkers and chain extenders are included
    5. Recalculate total reactive hydrogen equivalents
    6. Recalculate required NCO equivalents
    7. Recalculate TDI or MDI parts
    8. Compare the corrected formula against current production results
    Checklist for checking water equivalent weight in PU foam formula spreadsheet
    The first check is simple: water equivalent weight must be 9 in PU foam index calculations.

    Use the PolymerIQ Isocyanate Index Calculator

    Manual calculation is important because engineers should understand why water EW is 9. But in production, the calculation must also be checked quickly and consistently.

    The PolymersIQ Isocyanate Index Calculator can help verify whether your formula is using the correct equivalent weights and delivering the intended index.

    Use it to check water equivalent weight, total reactive hydrogen equivalents, required NCO equivalents, TDI or MDI parts, actual running index, and the effect of correcting EW errors.

    Open the Isocyanate Index Calculator →

    For the complete equivalent weight calculation guide, read Equivalent Weight in Polyurethane Foam: Complete Calculation Guide.

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

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

    FAQs

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

    Water has a molecular weight of 18 g/mol, 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. Equivalent weight measures the mass per reactive equivalent — not the mass per molecule — so the divisor matters.

    How does water actually consume two NCO groups?

    The reaction happens in two stages. First, water reacts with one NCO group to form an unstable carbamic acid that releases CO₂ (the blowing gas) and forms a primary amine. Second, the amine reacts with another NCO group to form a urea linkage. The result: one water molecule consumes two NCO groups.

    What happens if I use water EW = 18 by mistake?

    Using 18 cuts the calculated water equivalents in half. The total reactive hydrogen equivalents become wrong, the calculated isocyanate demand becomes wrong, and the actual running index can be much lower than the formula sheet shows. The foam may still rise but will likely be under-indexed.

    What does under-indexed foam look like in production?

    Common symptoms include softer foam than expected, ILD below target, poor compression set, weak recovery, slower or weaker cure, tacky feel during early cure, and poor aging performance. The foam may rise normally, which is why this error often stays hidden for a long time.

    Can this error explain unexplained compression set failures?

    Yes. If water EW is wrong and the foam is under-indexed, crosslink density is lower than designed, which directly affects compression set, recovery, and aging stability. Compression set problems that don’t respond to catalyst or silicone changes should trigger an EW audit.

    How do I check if my formula has this mistake?

    Open your formula sheet and find the equivalent weight value used for water. If it shows 18 instead of 9, the calculation is wrong. Also check the equivalents formula: it should be water parts ÷ 9, not water parts ÷ 18.

    Should I just change the water EW cell from 18 to 9?

    No — that alone is not enough. After correcting water EW, the entire index must be recalculated, and the isocyanate quantity may need to change as well. Changing only the water EW cell without recalculating the rest of the formula may create new imbalances.

    Why has this mistake stayed in some formula sheets for years?

    Because the formula sheet looks internally consistent. The index cell shows the target value, the math is formatted correctly, and the spreadsheet has been used for a long time. Troubleshooting usually focuses on machines and process variables, and the equivalent weight layer is rarely audited.

    Does this rule apply to flexible foam, rigid foam, and elastomers?

    Yes. Water has the same chemistry — two reactive hydrogens, two NCO groups consumed — regardless of the polyurethane system. Water EW = 9 applies to flexible slabstock, HR foam, rigid foam, elastomers, and any PU system that uses water as a blowing agent or reactive component.

    Does the water purity or temperature change the equivalent weight?

    No. The equivalent weight comes from the reaction stoichiometry, not from physical conditions. As long as the water is participating in the standard PU blowing reaction, EW = 9 is the correct value to use.

    Key Takeaways

    The equivalent weight of water in polyurethane foam is 9, not 18.

    Water has a molecular weight of 18, but it has two reactive hydrogens involved in the isocyanate reaction sequence:

    Water EW = 18 ÷ 2 = 9 g/eq

    Using 18 instead of 9 cuts the calculated water equivalents in half. This can severely corrupt the isocyanate index calculation and cause the actual running index to be much lower than the formula sheet suggests.

    The foam may still rise and look normal, but it can show soft hardness, poor compression set, weak recovery, and confusing production behaviour.

    Every PU foam formulation spreadsheet should be checked to confirm that water equivalent weight is entered as 9. A single wrong number can silently damage the entire stoichiometric calculation.

    Conclusion

    If your foam is consistently soft, failing compression set, or responding unpredictably to catalyst and process adjustments, the problem may not be the machine.

    It may be the equivalent weight foundation inside the formula sheet.

    PolymersIQ can help audit your formulation, verify water EW, recalculate the true index, and identify whether a hidden stoichiometric error is affecting production.

    To get accurate support, please share:

    • A screenshot or copy of your current formula sheet (with EW values)
    • Polyol OHV and isocyanate %NCO values currently in use
    • Water level and any crosslinkers or chain extenders
    • Target index and actual foam properties (ILD, compression set, density)
    • Description of the production issue you are facing

    Contact PolymerIQ for a stoichiometric formulation audit →

  • 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 →