Epoxy Mix Ratio by Volume vs Weight: Why Density Matters
Updated 2026-10-19
Every two-part epoxy has a mix ratio — for example 1:1 or 2:1 resin to hardener. That number is useless if you do not know whether it is by volume or by weight. Mixing the wrong way is one of the most common causes of soft spots, sticky surfaces, and incomplete cures. This guide explains the difference, why density matters, and how to measure correctly for your kit.
When you already know the total volume you need, the epoxy resin calculator can split Part A and Part B for common volume ratios. Always confirm the measuring method on your product label.
What "mix ratio" means
A ratio of 2:1 means two parts resin (Part A) to one part hardener (Part B). The critical question is: two parts of what?
- By volume: measure with graduated cups, marked mixing pots, or calibrated pumps. Two cups of A plus one cup of B.
- By weight: measure on a digital scale in grams or ounces (weight). Two grams of A for every one gram of B, if the ratio is 2:1 by weight.
These are not the same measurement. Resin and hardener almost always have different densities, so a cup of A does not weigh the same as a cup of B.
Why density makes volume ≠ weight
Density (or specific gravity on many data sheets) is mass per unit volume — often listed as g/cm³ or g/ml. If Part A is denser than Part B, the same volume of A weighs more. The following numbers are illustrative only — always use your product's TDS:
- Suppose Part A density is about 1.15 g/ml and Part B about 0.98 g/ml
- 1:1 by volume: equal milliliters of each → unequal grams
- 1:1 by weight: equal grams of each → unequal milliliters
If the manufacturer specifies 1:1 by volume and you weigh equal grams instead, you have shifted the chemistry. The reverse is also true. Off-ratio mixes may stay tacky, cure soft, or cure brittle.
How to tell which method your kit uses
- Read the front label and the technical data sheet.
- Look for phrases like "mix 2:1 by volume" or "mix 100:45 by weight".
- Pump kits and dual-cartridge systems are usually set up for a volume ratio.
- Kits that emphasize a kitchen scale and gram charts are usually weight-based.
If the packaging is ambiguous, do not guess — check the TDS/SDS from the manufacturer or contact them. Product-specific instructions override any general guide.
Worked example: volume ratio
You need 600 ml of mixed epoxy for a small pour. The kit is 2:1 by volume.
- Total parts = 2 + 1 = 3
- Part A = 600 × (2/3) = 400 ml
- Part B = 600 × (1/3) = 200 ml
Measure with volume-marked cups (or convert: 400 ml ≈ 13.5 fl oz, 200 ml ≈ 6.8 fl oz). Do not switch to equal weights unless the TDS says the weight ratio equals that volume split for this product.
Worked example: weight ratio
Same 600 ml target, but the kit is 100:45 by weight (resin:hardener). First convert volume to approximate mass using the mixed density from the TDS. If mixed density is about 1.1 g/ml:
- 600 ml × 1.1 ≈ 660 g mixed epoxy
- Parts: 100 + 45 = 145
- Part A = 660 × (100/145) ≈ 455 g
- Part B = 660 × (45/145) ≈ 205 g
Tare the cup on the scale, add A to the target, then add B to the combined target. Mix thoroughly. Density values vary by product — enter your TDS density in the epoxy calculator when estimating weight from volume.
Can you convert volume ratio to weight ratio?
Only if you know the density of each part (not just the mixed density). Rough conversion:
Weight of A = volume_A × density_A
Weight of B = volume_B × density_B
Weight ratio A:B = weight_A : weight_B
Without both densities, you cannot accurately convert. Do not invent densities. If the manufacturer publishes both a volume and a weight ratio, use the pair they provide for that product.
Pumps, cups and scales
- Volume pumps: convenient for repeated mixes; prime them and confirm they dispense the stated ratio before a critical pour.
- Graduated cups: read at eye level; account for the meniscus; use clean cups rated for epoxy.
- Digital scale: essential for weight ratios; prefer grams for small batches; tare between additions as your workflow requires.
For tiny batches, measurement error is a larger percentage of the total — weigh when possible if the kit allows it, and mix a little extra rather than scraping every last drop from an inaccurate cup.
Temperature and mixing technique
Cold resin is thicker and harder to measure cleanly; warm resin flows faster and may shorten working time. Keep resin, hardener, and the workspace in the manufacturer's recommended temperature range. Mix slowly and scrape the sides and bottom so unmixed material does not leave soft spots. Mixing folds in air — see how to fix epoxy bubbles for prevention tips.
Planning total volume first
Ratio math only splits what you already need. Size the project first:
- Coatings and tabletops → tabletop calculator
- Thick fills and layers → deep pour calculator
- River gaps → river table calculator
- Odd molds → mold volume calculator
Then apply the correct A/B method for your kit.
Common mistakes
- Assuming 1:1 always means equal cups. Only if the kit is 1:1 by volume.
- Using kitchen "ounces" without clarifying weight vs fluid ounces. They are different units.
- "Eyeballing" the hardener. Small ratio errors ruin cures.
- Mixing leftover A from one brand with B from another. Systems are formulated as pairs.
- Ignoring pot life. Large batches heat up faster; mix only what you can place in time.
Safety notes
Follow the SDS for ventilation, PPE and first aid. Uncured epoxy and hardeners need careful handling and disposal per local rules and the manufacturer's guidance.
Summary
Volume ratios and weight ratios are different measuring systems because Part A and Part B usually differ in density. Read the TDS, measure the way the product specifies, and size total volume with a calculator before you split A and B. Use the epoxy resin calculator for project volume and common volume-ratio splits — then verify against your label before you pour.