Quick answer: use the ratio exactly as your kit states it, by volume if it says volume and by weight if it says weight. The two are not interchangeable, because the resin and hardener usually have different densities. A kit that mixes 1:1 by volume might be something like 1.1:1 or 1.2:1 by weight, and only the manufacturer's data sheet tells you the right figure.
Why the ratios differ
Volume measures how much space a liquid takes up. Weight measures how heavy it is. If part A is denser than part B, equal volumes won't weigh the same.
Example with made-up densities: if resin weighs 1.15 g per ml and hardener weighs 0.95 g per ml, then 100 ml of each weighs 115 g and 95 g. The 1:1 volume ratio becomes about 1.21:1 by weight.
Mixing 1:1 by weight with that kit would give you too little resin and too much hardener.
Measuring by volume
- Use graduated mixing cups with clear markings
- Pour each part to the line at eye level, not looking down from above
- Use separate cups for each part, then combine, so a mistake in one doesn't ruin both
- Account for what clings to the cup: pour slowly and scrape
Volume measuring is simple for small batches but gets less accurate with thick hardeners that cling to cup walls.
Measuring by weight
- Use a digital scale that reads to at least 1 gram
- Tare the scale with the empty cup, pour part A, tare again, then pour part B
- Only use weight if your kit gives a weight ratio, or the manufacturer publishes one
Weighing is usually more accurate, especially for large batches and small amounts.
What happens if the ratio is off
Epoxy isn't like paint, where adding more hardener speeds things up. The two parts react in a fixed proportion. Too much of either part leaves some unreacted, which can cause:
- Soft or tacky spots that never fully cure
- Cloudiness or a greasy surface
- Weaker, more flexible cured resin
Small errors of a few percent may not show, but larger ones often do.
Mixing tips that matter as much as the ratio
- Mix for the full time on the label, often a few minutes, scraping the sides and bottom.
- Double-pot larger batches: mix in one cup, pour into a second cup and mix again, to catch unmixed resin from the walls.
- Mix only what you can use within the working time.
Splitting your total
The epoxy resin calculator splits your total volume into part A and part B for 1:1 and 2:1 volume ratios. If your kit uses a weight ratio, use the total from the calculator, then split it by weight using the manufacturer's numbers.
Worked example: splitting a batch
You need 32 fl oz of mixed epoxy for a tray.
With a 1:1 by volume kit: 16 fl oz of part A and 16 fl oz of part B.
With a 2:1 by volume kit: 21.3 fl oz of part A and 10.7 fl oz of part B.
With a weight ratio: use the manufacturer's numbers. If a kit states 100:45 by weight and you're making about 1,000 g in total, that's about 690 g of part A and 310 g of part B (1,000 × 100 ÷ 145 and 1,000 × 45 ÷ 145).
The smaller the batch, the bigger the effect of a small measuring error. For 30 ml of mixed epoxy, a 2 ml mistake is nearly 7% of the batch, enough to cause soft spots. For small projects:
- Use a scale reading to 0.1 g if your kit has a weight ratio
- Use syringes or small graduated cups for volume ratios
- Mix a slightly larger batch than you need, since leftover resin is cheaper than a failed piece
How to check a cure
After the stated cure time:
- The surface should be hard and not tacky to the touch
- A fingernail shouldn't leave a dent
- There should be no greasy or cloudy patches
If one area stays soft, it was usually unmixed resin scraped from the side of the cup. If the whole piece is soft, the ratio or temperature was off.
Fixing a bad mix
Soft or tacky areas don't cure later. Scrape out the soft resin, clean the area with the solvent your manufacturer recommends, and pour a fresh, correctly measured batch. A thin, fully cured layer over a tacky one rarely fixes the problem underneath.
Pigments and additives
Pigments, mica powders and alcohol inks add a small amount to the mix. Most manufacturers recommend keeping additives to a small percentage of the total so they don't upset the cure. Add them after measuring parts A and B, not as part of either.