Epoxy Resin Ratio Calculator

How It Works

Two-part epoxy cures through step-growth polymerization between an epoxide-containing resin (Part A) and a hardener that carries amine or anhydride functional groups (Part B). Every epoxide site needs a matching amine hydrogen to react with, so the stoichiometry is fixed at manufacture and printed on the data sheet as a mix ratio — often 1:1, 2:1, or 3:1. Deviating from that ratio by more than a few percent leaves unreacted molecules in the cured piece, which shows up as tackiness, cloudy patches, or a soft top layer that never fully hardens.

Manufacturers publish two different kinds of ratio and it matters which one you use:

• By weight — measured on a scale, in grams. Density-independent, so a 2:1 by weight always means two grams of A for every gram of B.
• By volume — measured in graduated cups, in ml. Depends on the density difference between Part A and Part B; a 2:1 by volume is not the same as a 2:1 by weight.

The calculator converts between the two using published Part A and Part B densities from the WestSystem Epoxy Technical Manual and ArtResin coverage charts. For volumetric target inputs (Length × Width × Depth), the tool computes displaced volume in cm³, converts to mixed-mass in grams using the combined resin density (typically 1.06-1.15 g/cm³ for common table-top epoxies), then splits the mass into Part A and Part B according to the manufacturer's mix ratio.

Curing is exothermic — the reaction releases heat — so a mix that stays within the safe pour depth (roughly 6 mm for standard table-top epoxy, 25 mm for slow-cure deep-pour formulas per the Ecopoxy Liquid Plastic FlowCast data sheet) will cure clear. Exceeding those depths triggers thermal runaway and is the concern of the separate resin-exotherm tool.

How to Use This Calculator

1. Select your resin system — choose the mix ratio (1:1, 2:1, 3:1, or custom) and whether the manufacturer specifies by weight or by volume.
2. Choose input mode — enter mold dimensions (L × W × D in mm) for a volumetric target, or a known total mixed weight if you already know how much you need.
3. Set the combined density — leave the default (1.10 g/cm³) for typical table-top epoxy, or override with the exact value from your product's data sheet.
4. Read the Part A and Part B weights — the calculator reports each in grams; round to the nearest 0.1 g on a digital scale.
5. Add a mix-loss buffer — small batches (under 100 g) lose 5-10% of the resin stuck to the cup and stir stick; the calculator can add a 10% overage if you enable it.
6. Export the batch card — save a PDF with the mix ratio, target weights, and pour date for your record.

Real-World Use Cases

River table pour

A woodworker pours a 1200 × 400 × 40 mm river-table channel in slow-cure epoxy at a 2:1 by-weight ratio. Volume works out to about 19,200 cm³, and at a combined density of 1.10 g/cm³ the total mixed weight is roughly 21,120 g. Split 2:1 gives 14,080 g Part A and 7,040 g Part B. The 40 mm depth stays under the manufacturer's 25 mm per-layer limit only if the pour is split across two 20 mm lifts — the calculator flags the depth against the safe limit so the woodworker knows before committing.

Small jewelry casting

A jeweler fills a 30 mm × 4 mm bezel mold with a fast-cure 1:1 epoxy. Displaced volume is about 2.8 cm³, so the total mixed mass is roughly 3.1 g and each part weighs 1.55 g. A 10% waste buffer bumps the mix to 3.4 g total, still tiny — but the calculator confirms the amount rather than eyeballing a jewelry-scale ratio where a 0.5 g error is a 30% swing.

Coating a serving tray

A hobbyist top-coats a 500 × 300 × 4 mm cutting board with art resin on a 2:1 by-volume system. Volume is 600 cm³; at a combined density of 1.12 g/cm³ the mixed mass is 672 g. The by-volume-to-by-weight conversion uses the ArtResin published densities (Part A ~1.15 g/cm³, Part B ~1.05 g/cm³), so the actual split is not a clean 2:1 by weight. The calculator reports 452 g A and 220 g B — off from the 448 g / 224 g a naive 2:1 by-weight split would give.

Tips & Safety Notes

• Weigh, do not measure by eye. A digital scale accurate to 0.1 g is the cheapest way to prevent tacky cures.
• Mix at room temperature. Cold Part A (below 18 °C) is thick and traps bubbles; warm the bottle in a water bath before mixing.
• Scrape the sides and bottom of the cup during mix. Unmixed pockets on the wall of the cup are the most common cause of soft spots.
• Wear nitrile gloves and eye protection. Amine hardeners are skin sensitizers; repeated skin contact causes allergic reactions.
• Discard the mixing cup outdoors. The residual resin in the cup exotherms as it cures and can generate enough heat to melt a plastic trash can lid.

Limitations & What This Tool Cannot Do

• Volume vs weight ratio. The tool converts between the two using published Part A and Part B densities; if your product uses non-standard fillers or heavy pigments, the density can drift and the conversion is approximate. When the two options disagree by more than ~2%, follow the manufacturer's data sheet.
• No mix-loss allowance in the base number. The theoretical mixed weight assumes 100% transfer into the mold; small batches lose 5-10% stuck to the cup and stir stick. Add a 10% overage for small pours.
• Not exotherm-safe on deep pours. The tool uses only the manufacturer's stated mix ratio; use the Resin Exotherm Calculator separately to check whether your pour depth stays under the thermal runaway limit.

FAQ

What is the difference between a resin's volume ratio and its weight ratio?

Volume ratios are measured in graduated cups; weight ratios are measured in grams on a scale. Because Part A and Part B usually have different densities, a 2:1 by volume is not the same as a 2:1 by weight. Always check which one your product specifies and stick to that method.

Why do resin manufacturers publish different mix ratios (1:1, 2:1, 3:1, etc.)?

The ratio depends on the specific epoxide and hardener chemistry — each product has a fixed stoichiometric ratio that fully consumes both parts. A 1:1 formula uses a hardener that is chemically balanced to the resin at equal weight; a 2:1 uses a lighter hardener that needs half the mass to react with all the epoxide sites. There is no universal ratio.

What happens if I mix the wrong Part A / Part B ratio?

Even a 5% deviation leaves unreacted molecules in the cured piece. You will see soft spots, tacky patches, cloudy areas, or a top layer that never hardens. Underdosing Part B is worse than overdosing — extra hardener eventually reacts with atmospheric moisture, while extra resin has nowhere to go.

Can I mix different brands of Part A and Part B together?

No. Even products marketed as compatible have subtly different amine profiles and ratio expectations. Mixed-brand batches almost always cure poorly. Buy Part A and Part B from the same manufacturer and the same product line.

How do I convert between fluid ounces and grams for resin mixing?

Multiply fluid ounces by 29.57 to get milliliters, then multiply milliliters by the product's density in g/cm³ to get grams. For common table-top epoxy at 1.10 g/cm³, one fluid ounce is about 32.5 g of resin. The calculator handles the conversion automatically once you enter dimensions in metric or imperial.

Why is my resin still tacky after 48 hours?

Most commonly the mix ratio was off — 5% deviation is enough to prevent full cure. Second most common is cold cure (below 18 °C ambient). Third is inadequate mixing that leaves unreacted pockets. Once a batch is tacky, sanding it off and re-pouring a thin fresh layer is usually the only remedy.

Related Tools

• Resin Exotherm Calculator — Check thermal runaway risk for deep pours
• Wax-to-Metal Casting Calculator — Volume/weight conversion in a different material system
• Candle Wax & Fragrance Calculator — Adjacent mold-and-pour chemistry

References

• WestSystem Epoxy Technical Manual (westsystem.com/technical-info) — mix ratios, density values, and cure schedules
• ArtResin published density and ratio charts (artresin.com/pages/coverage-calculator) — density and coverage tables used for by-volume conversions
• Ecopoxy Liquid Plastic FlowCast product data sheet — deep-pour safe-depth limits

Reviewed by the Craft Calc Lab team on August 1, 2026.