Epoxy Mixing Ratios Explained: Getting Part A:B Exactly Right
Volume ratio and weight ratio are never the same number for a given product — mixing up the two is one of the most common, most avoidable cure failures.
A mix ratio like 2:1 means two parts resin (Part A) to one part hardener (Part B) — but that number only means something once you know whether it’s by volume or by weight. Resin and hardener have different densities, so the same product can be 2:1 by volume and a completely different ratio by weight. The Resin Mixing Ratio Calculator does the volume math for any batch size once you know your product’s ratio.
How to read the inputs
Mix Ratio (Part A : Part B)
- Enter the exact ratio from your product’s TDS — presets are provided for common ratios like 1:1, 2:1, 3:1, and 4:1
Total to Mix — or One Known Part
- Your full batch or kit size, in whichever unit your product is sold in (L, mL, gal, or kg)
- Or switch to “Start From One Part” when you only know how much Part A or B you have
Part A / Part B
- The exact amount of each component to measure out for that batch size, plus each one’s share as a percentage
Unit toggle
- Switch between liters, milliliters, gallons, and kilograms depending on how your product is labeled
Worked example
A product rated at a 2:1 ratio by volume, mixing a 3 gal batch: total parts = 2 + 1 = 3, so each part is 3 ÷ 3 = 1 gal. Part A (2 parts) = 2.00 gal, and Part B (1 part) = 1.00 gal. If that same product’s TDS instead listed a weight ratio — say 100:83 — those figures would be different, because resin and hardener don’t weigh the same per gallon. Always confirm which basis the number on the label actually refers to.
Starting from a known amount of one component
The other direction comes up constantly on real jobs: a third of the Part A pail is left, and you want to use it up without guessing at the hardener. The math is the ratio applied directly — with 10 L of Part A in a 2:1 system, Part B = 10 × (1 ÷ 2) = 5 L, for 15 L of mixed product. The calculator’s “Start From One Part” mode does exactly this: pick the component you know, enter the amount, and it returns the matching quantity of the other part plus the total.
Mixing by weight instead of volume
A digital scale is the most accurate way to measure a mix — but only with the weight ratio. Because resin and hardener have different densities, the volume ratio converts through each component’s specific gravity: weight ratio = (vol A × SG of A) : (vol B × SG of B). A 2:1 volume system with a 1.12 SG resin and 0.94 SG hardener is 2.24 : 0.94 by weight — roughly 100 : 42, nowhere near 2:1. Most TDS sheets publish both ratios; if yours does, skip the conversion and use the published weight figure. If you only have the volume ratio, do the SG conversion once and write the result on the pail.
Try it with your own ratio
Enter your product's Part A:B ratio and total batch size — the split updates instantly.
Full result gives exact Part A and Part B volumes for any batch size.
Open the live calculator →Common mistakes
- Assuming volume ratio and weight ratio are the same number — they almost never are, since Part A and Part B have different densities
- Adding extra hardener to try to speed up the cure — it doesn’t work, and causes yellowing and a weaker film instead
- Not scraping the sides and bottom of the mixing container, leaving unmixed resin that shows up as soft spots later
- Eyeballing a ratio instead of measuring precisely — even a small error compounds across a full batch
Frequently Asked Questions
Is a 2:1 ratio by volume the same as 2:1 by weight?
No, almost never. Resin and hardener have different densities, so a product with a 2:1 ratio by volume might be closer to 100:83 by weight. Always check which basis your product’s TDS actually specifies before measuring — the two are not interchangeable.
What happens if I add a little extra hardener to speed up the cure?
It backfires. Excess hardener doesn’t cure faster — it leaves unreacted amine in the film, which can cause amber discoloration, a brittle or weak surface, and a higher risk of blushing. It never speeds up the working cure the way people assume.
Why is my epoxy still soft or tacky after mixing to the correct ratio?
The most common cause with a correct ratio is incomplete mixing — resin left unmixed along the sides or bottom of the container. Scrape the walls and base of the pot while mixing, then switch to a clean container and mix again briefly (a "double bucket" mix) before pouring.
Should I measure by volume or by weight?
Weight, using a digital scale, is more accurate for larger or more sensitive batches, since it accounts for the density difference directly. Volume measurement is faster and fine for most standard coating work. Either is acceptable as long as you use the ratio your product actually specifies for that method.
How do I convert a volume ratio to a weight ratio?
Multiply each side of the volume ratio by that component’s specific gravity from the TDS: weight ratio = (vol A × SG of A) : (vol B × SG of B). A 2:1 volume system with a 1.12 SG resin and 0.94 SG hardener works out to roughly 100:42 by weight. If the TDS already publishes a weight ratio, use that directly instead of converting.
I only know how much of one component I have — how much of the other do I need?
Apply the ratio directly: with 10 L of Part A in a 2:1 system, you need 10 × (1 ÷ 2) = 5 L of Part B, giving 15 L mixed. The calculator’s "Start From One Part" mode does this for either component in any unit.