What this calculator works out
This calculator splits a total quantity into part A and part B at the stated ratio, reporting both.
Epoxy is less forgiving of ratio errors than most adhesives. It is a stoichiometric reaction — the two parts consume each other in a fixed proportion, and anything left over stays uncured.
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Before you mix
- Check whether the ratio is by weight or by volume. Many epoxies state both, and the numbers differ.
- Mix thoroughly for the stated time, scraping the container. Under-mixing is the most common cause of a soft cure.
- Working time falls sharply with temperature and with the size of the mix.
- Wear nitrile gloves and ventilate. Epoxy is a recognised skin sensitiser and repeated exposure carries permanent risk.
Polyester resin uses a catalyst in a small proportion, where adding a little more genuinely does speed the cure. Epoxy uses a hardener that is a reactant in a fixed proportion, where more does not accelerate anything and simply leaves unreacted material. Applying polyester habits to epoxy is a common and expensive mistake.
How this calculator works
A proportional split:
Part A = total × A parts ÷ (A parts + B parts)
Part B = total × B parts ÷ (A parts + B parts)Both figures are given so neither has to be inferred. Weighing onto a tared scale is more accurate than volumetric measurement for small quantities, provided the ratio is specified by weight.
Worked example: 200 grams at 2:1
Using the default figures — 200 g total at 2:1:
- Part A: 133.3 g
- Part B: 66.7 g
Common epoxy ratios include 2:1, 3:1, 4:1 and 5:1 by volume, and manufacturers frequently publish a different set of numbers for weight. Mixing a 2:1-by-volume product as 2:1 by weight can be several per cent out, which for laminating systems is enough to reduce final strength measurably even though the piece appears to cure normally.
Common mistakes
- Adding extra hardener to speed the cure. Epoxy is not a catalysed system; it does not work.
- Confusing weight and volume ratios. Manufacturers publish both and they differ.
- Under-mixing. Mix for the stated time and scrape the sides.
- Mixing in cold conditions. Below the stated minimum, epoxy may never cure fully.
- Skipping gloves. Epoxy is a sensitiser and the effect is permanent.
Frequently asked questions
Why can I not add more hardener?
Because epoxy cures by a reaction in which the resin and hardener consume each other in a fixed molecular proportion. Excess hardener has nothing to react with and remains as uncured material in the mix, reducing strength and sometimes leaving the surface permanently tacky. This differs fundamentally from polyester resin, where the catalyst is used in a variable small proportion.
How does temperature affect it?
Considerably. Epoxy cures faster when warm and slows dramatically when cold — below around 15°C many systems cure very slowly, and below their stated minimum they may not fully cure at all. Warming the components before mixing, and keeping the work warm during cure, is standard practice in cool conditions.
How long does epoxy take to cure?
Working time is typically minutes to an hour depending on the system; handling strength commonly comes in several hours; full cure takes a day to a week. Manufacturers publish all three and they are not interchangeable — a joint at handling strength is not ready to be loaded.
Is epoxy safe to use at home?
With appropriate care. Nitrile gloves, ventilation and avoiding skin contact are the requirements, because epoxy is a recognised skin sensitiser and sensitisation is irreversible. Sanding cured epoxy creates dust that should not be inhaled. HSE guidance on sensitisers applies directly, and reading the safety data sheet for the specific product is worthwhile.
Is what I enter stored?
No. Figures are processed in your browser and never transmitted or retained.
Related tools
References
- Health and Safety Executive — epoxy resins, skin sensitisation and control of hazardous substances
- GOV.UK — chemical labelling, safety data sheets and disposal
Sources are checked at publication and can change — how I choose and check references.