Is 1:1 epoxy the same by weight and volume?

Not always. If the two parts have different densities, 1:1 by volume is a little off 1:1 by weight. Check which one your label means.

Which is more accurate, weight or volume?

Weight, especially for small batches, because a scale is more precise than the lines on a cup. Both work well when you use the ratio your label gives for that method.

What is specific gravity on an epoxy data sheet?

It is the liquid's density compared with water. A specific gravity of 1.15 means 1.15 grams per milliliter, which is the number you need to convert between volume and weight.

Epoxy mix ratio: weight vs volume

A label that says 2:1 is only half the instruction. The other half is whether that means cups or grams, and mixing them up is a common cause of sticky, soft epoxy.

Why the two ratios differ

Resin and hardener are different liquids with different densities. The resin is often heavier per milliliter than the hardener. So 2 cups of resin and 1 cup of hardener weigh more than 2 to 1. If you put a volume ratio on a scale, you get too little of the lighter part.

A worked example

A product mixes 2:1 by volume. The resin is 1.15 g/ml and the hardener 0.95 g/ml. For 400 ml of resin and 200 ml of hardener, the weights are 400 x 1.15 = 460 g and 200 x 0.95 = 190 g. That is 2.42 : 1 by weight. Weighing out 2 : 1 instead would give about 17 percent too little resin for the hardener, enough to cause soft spots.

How to convert

Find each part's density on the technical or safety data sheet, often listed as specific gravity (the same number in g/ml). Then grams = ml x density, and ml = grams / density. Many manufacturers publish both ratios, which saves the math. The mix ratio calculator does the conversion if you enter both densities.

Measuring by volume

Use graduated mixing cups with clear markings, on a level surface, read at eye level. Pour each part into its own cup first if you are unsure, then combine. Volume measuring is quick for larger batches, where a line's width is a small error.

Measuring by weight

Use a digital scale that reads to at least 1 g (0.1 g for tiny batches). Put the empty cup on, zero it, pour Part A to its weight, zero again, then pour Part B. Weight is the most accurate way to mix small amounts.

Common questions

Is 1:1 epoxy the same by weight and volume?: Not always. If the two parts have different densities, 1:1 by volume is a little off 1:1 by weight. Check which one your label means.
Which is more accurate, weight or volume?: Weight, especially for small batches, because a scale is more precise than the lines on a cup. Both work well when you use the ratio your label gives for that method.
What is specific gravity on an epoxy data sheet?: It is the liquid's density compared with water. A specific gravity of 1.15 means 1.15 grams per milliliter, which is the number you need to convert between volume and weight.