Volume first, then mass, then the split
Casting resin is bought by weight, mixed by weight or volume depending on the product, and consumed by volume. Those three units have to be reconciled in a fixed order, and doing them out of order is what produces a cup of mixed resin that either falls short of the mould or leaves a hundred grams to throw away.
The order is: work out the volume of the cavity, multiply by the number of cavities, add an allowance for what stays in the cup, convert that volume to mass with the density of the mixed system, and only then split the mass between the two components. The split comes last because the ratio applies to the whole mix, not to the resin alone — a 2:1 ratio means two parts A to one part B, so A is two-thirds of the batch, not twice the batch.
Centimetres make this painless. One cubic centimetre is one millilitre, so a mould measured in centimetres gives a volume in millilitres with no conversion at all, and multiplying by a density in grams per millilitre gives grams directly. If you work in inches, divide cubic inches by 16.3871 to reach millilitres; the calculator shows that step.
Why ratio by weight and ratio by volume are different numbers
This is the part of resin mixing that causes the most ruined castings, and it is worth being precise about. A mix ratio is a statement about proportion, but proportion of what changes the number. If resin and hardener have different densities — and they usually do — then a 2:1 ratio by volume is not a 2:1 ratio by weight.
Take a system where Part A has a density of 1.20 g/mL and Part B has 0.80 g/mL. Mix 100 mL of each and you have a 1:1 volume ratio, but on the scale you have 120 g and 80 g, which is 1.5:1 by weight. Follow a volume ratio on a scale, or a weight ratio in graduated cups, and you are off-ratio by 50%. Epoxy that is off-ratio does not cure proportionally softer; the excess of one component has nothing to react with and stays in the casting permanently as an uncured, tacky, sometimes weeping phase. There is no fix after the fact.
So the basis selector above is not a convenience. Read your product's technical data sheet, find whether the stated ratio is by weight or by volume, set the selector to match, and measure the way the data sheet says. Enter separate component densities only if the data sheet gives them; leaving both equal to the mixed density makes the two bases agree, which is the correct behaviour when the components are of equal density.
The geometric part of the calculation is simpler. A rectangle is length × width × depth. A cylinder is π/4 × diameter² × height — note the diameter, since entering a radius quadruples the answer. A sphere is π/6 × diameter³. For anything else, water is more accurate than geometry: seal the mould, fill it, tip it into a jug.
Worked example: six 10 cm coasters, 1 cm deep, 2:1 by weight
You are casting six round coasters in a silicone mould. Each cavity is 10 cm across and you fill to 1 cm deep. The resin is a 2:1 by weight epoxy with a mixed density of 1.10 g/mL, and you allow 5% for cup residue. Resin costs $30 per kilogram.
- Volume of one cavity. π/4 × 10² × 1 = 0.7854 × 100 = 78.54 mL. In cubic inches that is 78.54 ÷ 16.387 = 4.79 in³.
- Batch volume. 78.54 × 6 = 471.24 mL, and with the 5% allowance 471.24 × 1.05 = 494.80 mL.
- Mass of the mix. 494.80 × 1.10 = 544.3 g.
- Split it. A is 2 parts out of 3: 544.3 × 2 ÷ 3 = 362.9 g of Part A. B is the remainder: 544.3 − 362.9 = 181.4 g of Part B.
- Check the split. 362.9 ÷ 181.4 = 2.00 ✓, and 362.9 + 181.4 = 544.3 ✓.
- Cost. 0.5443 kg × $30 = $16.33 for the batch, or $2.72 per coaster in resin alone.
To mix it in practice, zero the scale with the empty cup on it, pour Part A to 362.9 g, then keep pouring Part B until the scale reads 544.3 g. That is one weighing operation rather than two, and it removes the risk of mis-taring between components.
How to read the result
The primary figure is what to weigh out, not what ends up in the mould — the difference is the cup-residue allowance. Five per cent is realistic for a batch of a few hundred grams. For a 30 g mix in a small cup, the film left behind is a much larger share, and 10–15% is closer to the truth. Undershooting on a batch of castings is far more costly than overshooting, because a part-filled mould cannot be topped up seamlessly once the first pour begins to gel.
Sanity-check the volume per casting against something you can picture. A standard coaster of 10 cm by 1 cm is about 79 mL, roughly a third of a mug. A pendant blank might be 3 mL. If the calculator reports 800 mL for something you expected to be small, you have almost certainly entered a radius where a diameter was asked for, or millimetres in a field set to centimetres.
Watch the fill depth as well as the volume. Casting resins have a maximum single-pour depth on their data sheets, typically stated in inches or centimetres, because the curing reaction is exothermic and heat trapped inside a thick mass accelerates it further. Depth is what governs that, not total volume: a shallow 2-litre pour is far safer than a 200 mL pour in a deep narrow tube. If your depth exceeds the product limit, pour in lifts, as the epoxy coverage and flood coat calculator lays out for surface work.
Finally, treat the cost per casting as a floor rather than a cost of goods. It excludes pigment, glitter, inclusions, the mould's amortised cost, sanding consumables, polishing compound and the time — and for small jewellery pieces the resin is rarely the largest line.
Volume and resin weight for common casting shapes
| Piece | Shape and size | Volume (mL) | Volume (in³) | Resin at 1.10 g/mL |
|---|---|---|---|---|
| Pendant blank | 3 × 3 × 0.5 cm slab | 4.50 | 0.275 | 4.95 g |
| Ring blank | 2.5 cm disc, 1.5 cm deep | 7.36 | 0.449 | 8.10 g |
| Domino / bookmark | 5 × 2.5 × 0.6 cm slab | 7.50 | 0.458 | 8.25 g |
| Coaster | 10 cm disc, 1 cm deep | 78.54 | 4.793 | 86.39 g |
| Ashtray / trinket dish | 12 cm disc, 2 cm deep | 226.19 | 13.803 | 248.81 g |
| Sphere | 6 cm diameter | 113.10 | 6.902 | 124.41 g |
| Sphere | 10 cm diameter | 523.60 | 31.952 | 575.96 g |
| Small tray | 25 × 15 × 1 cm slab | 375.00 | 22.884 | 412.50 g |
Add your own cup-residue allowance on top. A silicone mould's cavity is often slightly larger than the nominal size quoted by the seller, so measure it if the batch matters.
Mistakes that cost a whole batch
- Entering a radius where the field asks for a diameter. This multiplies a cylinder's volume by four and a sphere's by eight. It is the single most common error in mould arithmetic.
- Mixing a weight ratio in graduated cups. Or a volume ratio on a scale. Whenever the components differ in density these give different proportions, and off-ratio epoxy never fully cures.
- Splitting the ratio wrongly. A 2:1 mix is two-thirds A and one-third B, not twice the total. Multiply the total by rA ÷ (rA + rB).
- Mixing too little to be mixable. Below about 20–30 g, cup residue and imprecise scales dominate. Weigh a larger batch and pour off what you need rather than mixing three tiny cups.
- Ignoring the maximum pour depth. Depth, not volume, governs exotherm. A deep narrow casting in a table-top resin can reach temperatures that crack it or scorch the silicone.
- Forgetting displacement from inclusions. Dried flowers, glitter, beads and cured pieces already in the mould take up volume, so you need less resin, not more. Subtract their volume if it is significant.
- Using a scale with 1 g resolution for a 20 g mix. A gram of error in 20 g is 5% off-ratio. Use a 0.1 g scale for small work.
The single-cup weighing method
Put the mixing cup on the scale and tare it. Pour Part A until the scale reads the Part A figure. Do not tare again — keep pouring, now with Part B, until the scale reads the total. One tare, two pours, no arithmetic under time pressure, and no risk of double-taring away a component. This works only for weight-basis ratios; volume-basis products need graduated cups or two separate weighings using each component's own density.
Where this sits in a casting workflow
Before you cast, you often have to make the mould. That is the same volume problem run in reverse, with the master pattern's displacement subtracted from the mould box — the silicone mould material calculator handles it. If your master came off a printer, the resin 3D print cost calculator covers the photopolymer side, where the resin is consumed by the printed volume plus supports rather than by a cavity.
Where the piece is jewellery with a metal component, the jewellery metal weight calculator does the same volume-times-density arithmetic for silver, gold and brass, and the numbers get large quickly because those densities are ten to twenty times resin's. The same density logic drives candle wax quantities too, though wax is quoted per container rather than per cavity.
For flat coating rather than casting — a tabletop, a bar top, an art panel — the volume comes from area times film thickness instead of from geometry, and the epoxy coverage calculator is the right tool. The two problems share a formula but not a workflow: coating is limited by self-levelling and by lift depth, casting by cavity volume and exotherm.
Key terms
- Mix ratio
- The proportion of resin to hardener, quoted either by weight or by volume. The two are the same number only when the components have identical densities.
- Specific gravity
- Density relative to water. Because water is 1.00 g/mL, a specific gravity of 1.10 is the same as a density of 1.10 g/mL for these purposes.
- Exotherm
- The heat released by the curing reaction. It rises with pour depth because thick sections cannot shed heat, which is why data sheets state a maximum single-pour depth.
- Cup residue
- Mixed resin that stays on the walls of the mixing cup and the stir stick. Roughly constant per mix, so it hurts small batches proportionally more.