Two-Part Epoxy and Resin Mix Ratios: How to Calculate by Weight vs. Volume

Two-part resins are unforgiving chemistry: the mix ratio is fixed by the formulation, and getting it wrong by even 10% can leave you with a sticky, under-cured part. The catch is that a ratio quoted "by volume" is not the same as "by weight," and mixing the two up is the most common batching error. Here is how to convert correctly and batch the right amount.

Why weight and volume ratios differ

Resin (Part A) and hardener (Part B) almost always have different densities. A 2:1 ratio by volume means two scoops of A to one of B — but if A is denser than B, those two volumes weigh more than 2× the hardener. Convert with the densities:

ratio_weight = (V_A · ρ_A) : (V_B · ρ_B)

Worked example 1 — converting a volume ratio to weight

A laminating epoxy is specified 2:1 by volume. The resin density is ρ_A = 1.10 g/mL and the hardener is ρ_B = 0.98 g/mL.

weight ratio = (2 × 1.10) : (1 × 0.98) = 2.20 : 0.98 = 2.24 : 1

So "2:1 by volume" is really 2.24:1 by weight. Set your scale to that — using 2:1 by weight here would starve the mix of resin by roughly 11% and leave it under-cured.

Worked example 2 — batching a finished volume

You need to fill a mold with 500 mL of cured material using the 2.24:1-by-weight epoxy above. Work in parts:

Step 1 — Total mass for the target volume

First find the blended density. With 2 volumes of A and 1 of B, the mixed density is the mass over the total volume:

ρ_mix = (2·1.10 + 1·0.98) / (2 + 1) = 3.18 / 3 = 1.06 g/mL
total mass = 500 mL × 1.06 g/mL = 530 g

Step 2 — Split by the weight ratio

Parts total 2.24 + 1 = 3.24.

Resin A = 530 × (2.24 / 3.24) = 366 g
Hardener B = 530 × (1 / 3.24) = 164 g

Weigh out 366 g of resin and 164 g of hardener, and you will have ~500 mL of correctly proportioned material — plus whatever you add for waste and pot-life margin.

Don’t forget pot life and loss

• Mix only what you can place within the pot life. Larger batches exotherm faster — a quart that gels in 20 minutes in a cup may have 8 minutes in a deep mass.
• Add a waste allowance. Material clings to the cup and stir stick; 5–10% extra is typical for small batches.
• Account for solids loss in coatings — solvent or volatile content evaporates, so the cured film is thinner than the wet volume. Track it with a loss-on-drying calculation.

By weight vs. by volume: quick comparison

Worked example 3 — back-calculating from what you have

Often you do not start from a target volume — you start with a partial can of resin and want to use all of it. Say you have 800 g of Part A on hand and the formulation is 100:45 by weight (a common epoxy spec). How much hardener do you weigh?

B = A × (45 / 100) = 800 × 0.45 = 360 g

Total batch is 1,160 g. If you instead have a fixed amount of hardener, invert the ratio: A = B × (100/45). The arithmetic is trivial; the discipline is never eyeballing — an off-ratio epoxy does not cure harder or softer in proportion, it can fail to cross-link at all.

Temperature, viscosity, and degassing

• Temperature drives pot life. Most epoxies roughly halve their working time for every 18°F (10°C) rise. Mixing at 90°F instead of 72°F can turn a 30-minute pot life into 12.
• Warm the resin, not the mixed batch, to lower viscosity for easier wet-out — warming after mixing just accelerates the exotherm.
• Degas clear castings under vacuum after mixing to pull entrained air; thin coatings usually self-release if you mix slowly and scrape the container sides.
• Scrape the cup and stick. Unmixed resin clinging to the walls is the most common cause of soft spots — transfer to a second clean cup (“double-potting”) for critical work.

Two-part chemistry rewards precision. Know your densities, convert your ratio honestly, weigh rather than pour, and batch only what the pot life will let you place — and the part cures exactly the way the formulator intended.