Polyurethane Stoichiometric Calculator

Calculate the isocyanate demand for a polyurethane system from NCO and OH equivalent weights. Supports polyol blends, isocyanate blends, water contribution, and adjustable NCO:OH index.

Isocyanate Demand at a Given Index

Isocyanate (pbw) = (NCO Eq. Wt ÷ OH Eq. Wt) × 100 × (Index ÷ 100)
Eq. weights in g/eq  |  pbw = parts isocyanate per 100 parts polyol by weight  |  Index 100 = exact NCO:OH = 1.0

Eq. Wt (polyol) = 56100 ÷ OH Value   or   1700 ÷ %OH
Eq. Wt (isocyanate) = 4200 ÷ %NCO
Blend Eq. Wt = Total weight ÷ Σ(weight ÷ component Eq. Wt) — same rule for polyol and isocyanate blends

Polyol / Active-Hydrogen Input

Enter Eq. Weight
From datasheet
From OH Value/%OH
mg KOH/g
Polyol Blend
Multiple polyols + water

Derive Equivalent Weight from Hydroxyl Content

OH Value (mg KOH/g)
%OH (weight %)
Also called Hydroxyl Number / Hydroxyl Value — same parameter, same units.

Eq. Weight = —

From polyol technical data sheet, or use the OH Value converter above.

Polyol Blend Components

Effective Blend Eq. Weight = —

Isocyanate Input

Enter Eq. Weight
From datasheet
From %NCO
Weight % NCO
Isocyanate Blend
Multiple isocyanates

Derive Equivalent Weight from %NCO

Eq. Weight = —

From isocyanate technical data sheet, or use the %NCO converter above.

Isocyanate Blend Components

Effective Blend Eq. Weight = —

100 = exact NCO:OH = 1.0. Rigid foams commonly run 105–115; waterborne systems often run higher to offset the competing water reaction.
Enter your actual batch size, or leave at 100 for pbw directly. For a polyol blend, enter the blend's total weight.
Enter both equivalent weights to check the NCO:OH index

Isocyanate Required

—

Mix Ratio (Polyol : Isocyanate)

Enter values

Total Mixed Weight

—

Practical Note

This is the theoretical stoichiometric starting point only. Any moisture in the polyol side reacts with isocyanate too — include it as a "water" blend component if it's more than trace, since it consumes NCO and generates CO₂ (a real concern in coatings and adhesives, not just foams). Running well above 100% index leaves unreacted NCO that can react with atmospheric moisture during cure, causing surface bubbling or CO₂ blistering in humid conditions. Confirm the actual %NCO and OH Value from the current lot's certificate of analysis — both drift batch to batch — and verify free NCO by titration before scaling a production batch.

Data Sources & References

  1. Oertel, G. (Ed.) — Polyurethane Handbook, 2nd Ed., Hanser, 1993
  2. Randall, D., Lee, S. (Eds.) — The Polyurethanes Book, Wiley, 2002
  3. Woods, G. — The ICI Polyurethanes Book, 2nd Ed., Wiley, 1990
  4. Ulrich, H. — Chemistry and Technology of Isocyanates, Wiley, 1996

Isocyanate demand: pbw = (NCO Eq. Wt / OH Eq. Wt) x 100 x (Index / 100). Equivalent weight from OH Value: Eq. Wt = 56100 / OH Value. Equivalent weight from %OH: Eq. Wt = 1700 / %OH. Equivalent weight from %NCO: Eq. Wt = 4200 / %NCO. Water Eq. Wt = 9 g/eq (18 g/mol ÷ 2 active hydrogens).

Built by PolymerDNA — more formulation references in the Formulae Masterbook.

Scroll to Top