Epoxy Resin Calculator
Resin and hardener amounts for a batch size, mix ratio, and waste allowance.
What it does
Epoxy is a two-part reactive system. The resin and hardener must be measured in the ratio the manufacturer specifies, because the chemical reaction consumes both parts together. A batch that is short on hardener stays soft or tacky; a batch with too much hardener also cures poorly and wastes expensive chemistry. The job of a batch calculator is to turn a target amount of mixed epoxy into two numbers you can actually measure.
Ratios appear in two forms. Volume ratios are common with pump systems and graduated cups, such as 5 parts resin to 1 part hardener. Weight ratios are more accurate for small batches and for products whose two parts have different densities. A product can be 5:1 by volume and 5.36:1 by weight, as West System 105/206 is. The two numbers describe the same cured material, but they are not interchangeable. Use the basis printed on the data sheet and do not convert one to the other without the supplied densities.
Mixing loss is real. A thin film clings to the cup, the stick, and the brush, and some epoxy stays behind when you pour. A waste allowance adds a percentage to the planned batch before the ratio is split, so the cup still holds enough usable material. Five to ten percent covers most small jobs. Large batches in warm shops may need less, while a batch spread over many small parts may need more.
This page returns the resin amount, the hardener amount, the total mixed batch including waste, and the ratio it used. The default metric batch is 300 ml at 5:1 by volume with 10 percent waste, which gives 330 ml total, 275 ml resin, and 55 ml hardener. The imperial default is 10 fl oz and gives 11 fl oz total, 9.17 fl oz resin, and 1.83 fl oz hardener.
The result is a measuring target, not a cure schedule. Pot life, working time, minimum temperature, and full cure come from the data sheet. Thickeners, fillers, pigments, and extenders change the mix and may change the ratio. Structural repairs, marine laminates, and load-bearing bonds should follow the manufacturer's instructions and an engineer's specification.
How to use
- Pick metric or imperial, then choose whether the ratio is by volume or by weight.
- Enter the amount of mixed epoxy you want before waste. A cup marked in milliliters or fluid ounces is the easiest starting point.
- Enter the resin and hardener parts exactly as printed on the product data sheet.
- Add a waste allowance. Start with 10 percent for a small batch and adjust after you see how much stays in the cup.
- Read the resin and hardener amounts, then measure each part into a clean container on a level surface.
- Mix for the full time stated on the data sheet, scrape the sides and bottom, and use the batch within its pot life.
Formula
- planned batch
- The amount of mixed epoxy you want before any waste allowance, in ml, g, fl oz, or oz.
- waste
- Percent added for material left in the cup, on the stick, or on the brush.
- resin parts
- Resin side of the mix ratio.
- hardener parts
- Hardener side of the mix ratio.
- total
- Planned batch plus waste, and the sum of the resin and hardener amounts.
- ratio
- The parts comparison used for the batch, shown with its volume or weight basis.
The denominator is the sum of both parts, not the resin part alone. For a 5:1 ratio, the resin fraction is 5/6 and the hardener fraction is 1/6. For a 2:1 ratio, the fractions are 2/3 and 1/3. For a 100:44 ratio, the fractions are 100/144 and 44/144. Keep both parts in the same unit. If the data sheet gives a volume ratio but you want to weigh the parts, use the weight ratio from the same sheet instead of assuming that 1 ml equals 1 g. The two components usually have different densities.
Worked example
Example: 300 ml batch at 5:1 by volume
A project needs about 300 ml of mixed epoxy. The product is 5 parts resin to 1 part hardener by volume, and the user wants 10 percent extra for mixing loss.
- Waste multiplier = 1 + 10 / 100 = 1.10.
- Total mixed epoxy = 300 x 1.10 = 330 ml.
- Total ratio parts = 5 + 1 = 6.
- Resin = 330 x 5 / 6 = 275 ml.
- Hardener = 330 x 1 / 6 = 55 ml.
- Check: 275 + 55 = 330 ml, and 275 / 55 = 5, so the ratio is correct.
- For a 10 fl oz imperial batch at the same ratio, the total is 11 fl oz, with 9.17 fl oz resin and 1.83 fl oz hardener.
Reference table
| Planned batch | Total mixed | Resin | Hardener |
|---|---|---|---|
| 100 ml | 110 ml | 91.67 ml | 18.33 ml |
| 250 ml | 275 ml | 229.17 ml | 45.83 ml |
| 300 ml | 330 ml | 275 ml | 55 ml |
| 500 ml | 550 ml | 458.33 ml | 91.67 ml |
| 1000 ml | 1100 ml | 916.67 ml | 183.33 ml |
Limits
- Uses only the ratio you enter. A 5:1 result is wrong for a 2:1 or 100:44 product.
- Does not model pot life, exotherm, working time, or minimum cure temperature.
- Waste is an estimate. It depends on the container, the mixing method, and how much material stays on the tools.
- Fillers, thickeners, and pigments can change the required ratio or the mixed volume. Follow the product data sheet.
Common mistakes
- Using a volume ratio when the data sheet gives a weight ratio, or the reverse.
- Adding extra hardener to make the batch cure faster. It usually makes the cured epoxy weaker.
- Measuring both parts in the same dirty cup without accounting for what stays behind.
- Mixing a batch that is too small for the pump or scale to measure accurately.
- Forgetting to scrape the sides and bottom of the cup, which leaves unmixed resin or hardener in the batch.
- Mixing a large batch in a deep cup, where the exotherm can shorten pot life and cause a rapid cure.
Try it yourself
Quick checks. Answers sit tucked under each question.
A 2:1 by volume epoxy needs 600 ml mixed, with 5 percent waste. How much resin and hardener?
Total = 600 x 1.05 = 630 ml. Resin = 630 x 2 / 3 = 420 ml. Hardener = 630 x 1 / 3 = 210 ml.
A 100:44 by weight epoxy needs 500 g with no waste. What are the two parts?
Total parts = 144. Resin = 500 x 100 / 144 = 347.22 g. Hardener = 500 x 44 / 144 = 152.78 g.
A 5.36:1 by weight epoxy needs 250 g with 10 percent waste. What are the two parts?
Total = 275 g. Resin = 275 x 5.36 / 6.36 = 231.76 g. Hardener = 275 x 1 / 6.36 = 43.24 g.
Questions
Is epoxy ratio by weight or by volume?
It depends on the product. Many marine epoxies list both, and the numbers can differ because the resin and hardener have different densities. Use the basis on the data sheet.
Can I add extra hardener to speed the cure?
No. The ratio is a chemical balance. Extra hardener does not have enough resin to react with, and the leftover can weaken the cure or leave an oily surface.
How much waste allowance should I use?
Ten percent is a reasonable start for a small batch. Increase it for a batch spread across many small molds or applied with a brush, and reduce it for a large batch poured directly from one cup.
Why does the total include waste but the ratio still look the same?
Waste is added before the split, so both parts grow by the same percentage. The ratio of resin to hardener stays the same.
Can I use this for polyester resin?
No. Polyester and vinylester resins use a catalyst at a much smaller percentage, and the calculation is different. This page is for two-part epoxy systems.
What if my scale reads only whole grams?
Make a larger batch so the hardener amount is well above the scale resolution. A 1 g resolution is too coarse for a batch that needs 3 g of hardener.
Sources
- West System: 105/206 Epoxy Technical Data Sheet
- System Three: General Purpose Epoxy Resin
- Huntsman: Araldite CW 5715 US and Aradur HY 5716 US Technical Data Sheet