What this calculator works out
This calculator splits a total mix into resin and hardener at the ratio you specify, reporting both components.
Casting resin cures by chemical reaction, not by drying. An incorrect ratio does not cure slowly — it cures incompletely, and the result stays soft or tacky permanently.
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Before you mix
- Check whether the manufacturer states the ratio by weight or by volume. The two are different and are not interchangeable.
- Mix only what you can pour within the pot life, which for many resins is a few minutes.
- Scrape the sides and bottom of the mixing container thoroughly. Unmixed resin on the walls is the most common cause of soft patches.
- Wear nitrile gloves and work with good ventilation. Repeated skin contact causes sensitisation, which is permanent.
Casting and laminating resins are recognised skin and respiratory sensitisers. Once someone becomes sensitised, subsequent exposure — even to a small amount — can produce a serious reaction, and the effect does not wear off. Gloves, ventilation and avoiding skin contact are not precautions for the careless; they are what prevents an irreversible change.
How this calculator works
A proportional split:
Resin = total × resin parts ÷ (resin parts + hardener parts)
Hardener = total × hardener parts ÷ (resin parts + hardener parts)Both components are reported, because giving one and leaving the other to be worked out is where mixing errors start. The ratio is echoed back so it can be checked against the container before pouring.
Worked example: 200 grams at 2:1
Using the default figures — a 200 g total mix at a 2:1 ratio:
- Resin: 133.3 g
- Hardener: 66.7 g
Weigh both onto the same scale, tared between them, rather than measuring one and estimating the other. Note that a 2:1 ratio by volume is not 2:1 by weight — resin and hardener have different densities, and a product specified by volume mixed by weight can be several per cent out, which for a sensitive system is enough to affect the cure.
Common mistakes
- Mixing by weight a product specified by volume, or the reverse. The densities differ.
- Adding extra hardener to speed the cure. It produces a weaker, incompletely cured result.
- Not scraping the container. Unmixed material on the sides causes soft patches.
- Mixing more than the pot life allows. Large volumes also generate heat and cure faster.
- Working without gloves and ventilation. Sensitisation is permanent.
Frequently asked questions
What happens if the ratio is wrong?
The mix cures incompletely. Too much hardener does not accelerate a good cure — it leaves unreacted material and a weaker, sometimes permanently tacky result. Too little leaves soft resin. Neither can be corrected after the fact; the piece has to be discarded and remade.
Why does a large mix get hot?
The curing reaction is exothermic, and a large mass retains its own heat, which accelerates the reaction further. A volume that would cure gently in a thin layer can smoke or crack in a deep pour. Manufacturers state maximum pour depths for this reason, and deep casts are built up in layers.
What safety equipment do I need?
Nitrile gloves — not latex, which many resins permeate — good ventilation or extraction, and eye protection when pouring. For sanding cured resin, a mask, since the dust is a respiratory irritant. HSE guidance on skin and respiratory sensitisers applies directly to this material.
How do I avoid bubbles?
Mix slowly to avoid folding air in, let the mixed resin stand for a minute where the pot life allows, and warm the resin slightly to reduce viscosity. A pressure pot is the reliable solution for clear casting and is what commercial casters use. A heat gun passed briefly over the surface releases surface bubbles.
Is what I enter stored?
No. Figures are processed in your browser and never transmitted or retained.
Related tools
References
- Health and Safety Executive — skin and respiratory sensitisers, resin safety and personal protective equipment
- GOV.UK — chemical labelling, safety data sheets and hazardous waste disposal
Sources are checked at publication and can change — how I choose and check references.