MEKP Catalyst Calculator
Get the right amount of MEKP hardener for your polyester resin, vinyl ester, or gelcoat. Enter how much resin you are mixing and your shop temperature, and the calculator gives you the dose in cc and fluid ounces, adjusted for the weather, so your batch gels on time and cures hard.
How the MEKP calculator works
MEKP (methyl ethyl ketone peroxide) is the catalyst that kicks off the cure in polyester and vinyl ester resins and gelcoat. The standard dose is about 1.25 percent of the resin volume at 77°F, and the calculator slides that up in cold weather and down in heat, because temperature changes how fast the resin gels:
- Resin amount. The volume of resin or gelcoat you are about to catalyze, in ounces, quarts, gallons, or mL.
- Shop temperature. Warmer resin gels faster, so it needs less catalyst. Colder resin needs more to cure in a reasonable time. Try to work between 60 and 90°F.
The result stays inside the safe 1 to 2 percent range that every resin manufacturer publishes. Below about 1 percent the resin can stay soft, and above 2 percent it can crack, smoke, or even self-ignite in a large mass.
MEKP catalyst FAQ
How much MEKP do I add to polyester resin?
About 1.25 percent of the resin volume at room temperature, which works out to roughly 12 cc (a bit under half a fluid ounce) per quart, or about 47 cc per gallon. Use more in cold weather and less in heat. Enter your batch above for the exact amount.
What happens if I use too much or too little catalyst?
Too little and the resin gels slowly or stays soft and tacky. Too much and it gels before you can work it, generates excess heat, and can crack, smoke, or discolor. Staying in the 1 to 2 percent range keeps the cure predictable. More catalyst does not make a stronger part.
Does temperature change how much catalyst I use?
Yes. Heat speeds the reaction, so warm resin needs less catalyst and cold resin needs more. The calculator adjusts the dose for your shop temperature. Below 60°F, warm the resin first, and above 90°F mix small batches and pour quickly.
How much hardener for a gallon of resin?
At the standard 1.25 percent room-temperature dose, about 47 cc, or roughly 1.6 fluid ounces, of MEKP per gallon. The calculator scales this for your temperature and batch size.
Can I use MEKP with epoxy resin?
No. MEKP is only for polyester resin, vinyl ester, and gelcoat. Epoxy uses a separate measured hardener mixed at a fixed ratio by volume or weight, not a small percentage of catalyst. For epoxy, use our resin calculator instead.
How long before polyester resin gels after adding MEKP?
At a standard dose and room temperature, working time is roughly 15 to 30 minutes, with a full cure over several hours. A warmer shop, a thicker pour, or more catalyst all shorten that, so mix only what you can use before it kicks.