Calculate the NCO/OH ratio of a polyurethane mix from NCO content and OH number, or find the isocyanate or polyol mass needed for a target index.

Use the supplier’s %NCO. Suggestions are theoretical monomer values; polymeric MDI is typically about 31–32%.

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Safety notice (isocyanates): For industrial/professional use. Isocyanates can cause respiratory sensitization and serious irritation. Before handling or mixing, read and follow the product Safety Data Sheet (SDS) and supplier technical guidance, use appropriate PPE (e.g., suitable gloves/eye protection and respiratory protection where required), and ensure adequate ventilation/engineering controls. If you’re unsure about exposure controls or safe handling, consult your supplier or a qualified industrial hygiene professional.

Related Calculators

NCO/OH Ratio Formula

The following formula is used to calculate the NCO/OH ratio for a given amount of NCO and OH equivalents.

R = ENCO / EOH

Variables:

  • R is the NCO/OH ratio
  • ENCO is the NCO equivalents (eq)
  • EOH is the OH equivalents (eq)

To calculate the NCO/OH equivalent ratio R, divide NCO equivalents by OH equivalents. A ratio of 1.05 corresponds to an NCO index of 105 under the percentage-index convention; neither value is a component mass ratio. This calculator uses equivalent weights of 4202 / NCO wt%, 56100 / OH number, or 1700 / OH wt%. These use 42.02 g per NCO equivalent and the disclosed rounded 17.00 g per OH equivalent.

Masses and reactive-group specifications must describe the same material basis, such as as-supplied solution or resin solids. This two-component model excludes water, amines and other reactive ingredients; their demand must be included separately using supplier guidance. PMA’s urethane calculations explains equivalent weights and reactive-group accounting.

What is an NCO/OH Ratio?

The NCO/OH ratio is a measure used in polymer chemistry to describe the ratio of isocyanate groups (NCO) to hydroxyl groups (OH) in a chemical reaction. This ratio is crucial in determining the properties of the resulting polymer, such as its flexibility, hardness, and durability. By adjusting the NCO/OH ratio, chemists can tailor the characteristics of the polymer to suit specific applications, such as coatings, adhesives, foams, and elastomers.

How to Calculate NCO/OH Ratio?

The following steps outline how to calculate the NCO/OH ratio.

  1. First, determine the NCO equivalents (ENCO).
  2. Next, determine the OH equivalents (EOH).
  3. Finally, calculate the NCO/OH ratio using the formula R = ENCO / EOH.
  4. After inserting the values and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

NCO equivalents (ENCO) = 10

OH equivalents (EOH) = 5

The example gives R = 10 / 5 = 2 : 1, or index 200 under the percentage-index convention. It is an arithmetic example, not a recommended formulation target.