MEKP Catalyst Calculator
How much MEKP hardener to add to polyester resin, vinyl ester, or gelcoat, adjusted for your shop temperature. The answer updates as you type.
- Free, no signup
- Dose by temperature
- cc and fl oz
- Stays in the safe 1 to 2% band
MEKP catalyst chart by temperature
MEKP needed for common batch sizes, in cc (mL). Use a graduated dispenser: drop sizes vary from bottle to bottle, so drops are not a reliable measure.
| Shop temp | Dose | 4 oz resin | 8 oz | 16 oz | 1 quart | 1 gallon |
|---|---|---|---|---|---|---|
| 60°F and below | 2.0% | 2.4 cc | 4.7 cc | 9.5 cc | 19 cc | 76 cc |
| 65°F | 1.8% | 2.1 cc | 4.3 cc | 8.5 cc | 17 cc | 68 cc |
| 70°F | 1.55% | 1.8 cc | 3.7 cc | 7.3 cc | 15 cc | 59 cc |
| 77°F (standard) | 1.25% | 1.5 cc | 3.0 cc | 5.9 cc | 12 cc | 47 cc |
| 80°F | 1.2% | 1.4 cc | 2.8 cc | 5.7 cc | 11 cc | 45 cc |
| 85°F | 1.1% | 1.3 cc | 2.6 cc | 5.2 cc | 10 cc | 42 cc |
| 90°F and above | 1.0% | 1.2 cc | 2.4 cc | 4.7 cc | 9.5 cc | 38 cc |
Percent by volume for laminating resin. 1 fl oz = 29.6 cc. Never go below 1% or above 2%. Gelcoat runs higher, about 2% at 77°F: switch the calculator to Gelcoat or see the gelcoat MEKP chart.
How the MEKP calculator works
MEKP (methyl ethyl ketone peroxide) starts the cure in polyester and vinyl ester resins and gelcoat. The standard dose is 1.25 percent of the resin volume at 77°F. The calculator raises the dose as the shop gets colder and lowers it as it gets warmer, because temperature controls how fast the resin gels.
- Resin amount. The volume of resin or gelcoat in this one batch: fl oz, pints, quarts, gallons, or mL.
- Resin or gelcoat. Gelcoat takes a higher dose than laminating resin: about 2 percent at 77°F, held between 1.5 and 2.5 percent.
- Shop temperature. Warm resin gels faster and needs less catalyst. Cold resin needs more to cure in a reasonable time. Work between 60 and 90°F when you can.
Below 1 percent the resin can stay soft. Above 2 percent it can crack, smoke, or even self-ignite in a large mass. More catalyst gives a faster gel, not a stronger part.
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.
How much MEKP do I add to gelcoat?
About 2 percent by volume at 77°F, roughly 19 cc per quart or 76 cc per gallon. Use a little more in a cold shop and a little less in heat, and stay between 1.5 and 2.5 percent. The gelcoat calculator also works out how much gelcoat the job needs.