Why is the mix ratio so important with resin?

Resin and hardener undergo a chemical reaction (cross-linking) to cure. The ratio is carefully formulated by the manufacturer so that every molecule of one component has the right amount of the other to react with. If the ratio is off — even by a small amount — the cured resin will be sticky, weak, brittle or not cure at all. Always measure accurately by weight or volume as specified on the product.

Should I measure by weight or by volume?

Check your resin's technical data sheet — some ratios are by volume, others by weight, and they are not the same thing because resin and hardener have different densities. Measuring by weight is generally more accurate than by volume, especially for smaller quantities. A cheap digital scale accurate to 0.1g makes a significant difference to results.

What is pot life and how long do I have to work?

Pot life (also called working time or open time) is the period from mixing until the resin becomes too viscous to pour or manipulate effectively. It is measured from the moment mixing begins. As a general rule, work within 75% of the pot life — once the mixture starts to warm or thicken noticeably, stop adding pigment or making adjustments and pour immediately.

Why does temperature affect pot life and cure time?

The curing reaction is temperature-dependent. Warmer temperatures speed up the chemical reaction, shortening pot life and cure time. Cooler temperatures slow everything down. As a rough guide, every 10°C rise approximately halves the pot life and cure time. Working in a cold room (below 15°C) can cause resin to cure incompletely or remain permanently tacky.

What causes the exothermic (heat) reaction in resin?

Cross-linking is an exothermic chemical reaction — it releases energy as heat. In small, thin pours this heat dissipates harmlessly. In large or deep pours, the heat builds up in the mass of resin, accelerating the reaction further and potentially causing cracking, yellowing, smoking or even fire. Deep-pour epoxy resins use slower-reacting chemistry to reduce this risk and allow thicker pours safely.

CraftCalcs

Resin Mixing Calculator — Hardener Ratios & Pot Life

Calculate exact hardener amounts, total mixed volume, adjusted pot life and exotherm risk for epoxy and polyester resin work.

Resin Volume

The amount of resin (Part A) you want to use — not the total mixed volume

Mix Ratio (Resin:Hardener)Check your resin packaging carefully — ratios vary significantly between products and are usually by volume

Pot Life

The pot life stated on your resin packaging at the reference temperature — usually 20–25°C

Working Temperature

Actual room temperature where you are working — significantly affects pot life and cure time

Resin TypeSelect the type of resin — this affects exotherm risk assessment and cure time estimates

How We Calculate This

This calculator works out the correct amount of hardener to add to your chosen resin volume based on the manufacturer mix ratio, then estimates how long you have to work with the mix.

Hardener volume

Hardener volume = Resin volume ÷ Resin parts × Hardener parts

For example, a 2:1 ratio with 200ml resin: hardener = 200 ÷ 2 × 1 = 100ml. Total mixed = 300ml.

Pot life adjustment

Pot life is adjusted for temperature using the Arrhenius approximation: every 10°C above 20°C approximately halves pot life. Large volumes also reduce effective working time due to the exothermic mass effect.

Safe working time = 75% of adjusted pot life

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Always verify quantities before purchasing materials.