Enter the gelcoat amount, catalyst amount, and the ratio into the calculator to determine the missing value.
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Gelcoat to Catalyst Ratio Formula
The calculator uses one of three formulas depending on the mode you choose.
Volume or weight mode (find catalyst from gelcoat amount):
Catalyst = Gelcoat × (Percent ÷ 100)
Check mix mode (find percent from what you already poured):
Percent = (Catalyst ÷ Gelcoat) × 100
Ratio form used in the result panel:
Ratio = 100 ÷ Percent
- Gelcoat — amount of gelcoat resin, in volume (mL, fl oz, qt, gal) or weight (g, oz, lb).
- Catalyst — MEKP hardener, in the same measurement type as the gelcoat.
- Percent — MEKP catalyst rate, typically 1% to 2%.
- Ratio — parts gelcoat per 1 part catalyst (for example, 50:1 at 2%).
The calculator treats volume-to-volume and weight-to-weight as the same percent, since gelcoat and MEKP have similar densities (close to 1.0 g/mL). For drop estimates, 1 mL is approximated as 20 drops. Always confirm the final rate against the manufacturer’s product data sheet.
Common Mix Rates and Quick Reference
MEKP rate depends on shop temperature, batch size, and the gelcoat product. The table below shows typical ratios and their effect.
| MEKP % | Ratio | Use case |
|---|---|---|
| 1.0% | 100:1 | Hot shop, large batch, slow cure |
| 1.25% | 80:1 | Warm shop, normal batch |
| 1.5% | ~67:1 | Standard rate, 70 to 80°F |
| 1.75% | ~57:1 | Cool shop, small batch |
| 2.0% | 50:1 | Cold shop, fast cure (max for most) |
Quick catalyst amounts at 1.5% and 2% for common gelcoat batches:
| Gelcoat | MEKP at 1.5% | MEKP at 2.0% |
|---|---|---|
| 1 fl oz | 0.44 mL (~9 drops) | 0.59 mL (~12 drops) |
| 8 fl oz (1 cup) | 3.55 mL | 4.73 mL |
| 1 qt | 14.2 mL | 18.9 mL |
| 1 gal | 56.8 mL (~1.9 fl oz) | 75.7 mL (~2.6 fl oz) |
Example and FAQ
Example. You have 1 quart of gelcoat and want to mix at 1.8%. Quart is 946.35 mL. Catalyst = 946.35 × 0.018 = 17.03 mL. That is about 0.58 fl oz, or roughly 3.5 teaspoons.
What happens if I use too little MEKP? Below about 1%, the gelcoat may stay tacky, cure soft, or never fully harden. Sand-able cure can take much longer.
What happens if I use too much? Above 2.5 to 3%, pot life shortens fast, the gelcoat can pre-gel in the cup, and the cured film may be brittle, yellowed, or porous.
Does temperature matter? Yes. Warmer shops cure faster, so use a lower MEKP rate. Cool shops need more catalyst to hit the same gel time. Below 60°F, most gelcoats should not be sprayed at all.
Can I measure by weight? Yes. Gelcoat and MEKP are close enough in density that a 2% volume mix and a 2% weight mix give effectively the same result.