Resin Calculator
Work out the mould volume in millilitres, multiply by the number of cavities plus a mixing-loss allowance, and split the total by the resin's A:B ratio — by weight or by volume, whichever the label specifies. This resin calculator handles box, cylinder, sphere and dome moulds, converts ml to grams at typical epoxy density, and tells you exactly how much of each part to mix.
Check your resin's label — the ratio is not interchangeable
Resin left on cups and sticks — 10% is safe
Mix
Mixed density assumed 1.15 g/ml (typical casting epoxy). Weigh both parts on a digital scale — the 2:1 ratio only works measured the way the label says (weight).
▸How this was worked out
mould volume = 300 ml per cavity
with 1 cavities and 10% mixing loss: 300 × 1 × 1.10 = 330 ml
total weight = 330 ml × 1.15 g/ml = 379.5 g
A = 379.5 × 2/3 = 253 g · B = 126.5 g
Mix slightly more than the math says — you cannot pour the film left on the cup. Pot life starts at mixing; work in batches you can place within 20–40 minutes.
Shopping list
- Epoxy resin (2:1 by weight)A 253 g + B 126.5 g330 ml
- Nitrile gloves + mixing cupstwo cups per batch: mix in one, pour from a clean one1 set
CraftCalc · Resin pour sheet
Rectangular mould · 1 cavities of 300 ml · 2:1 by weight
Mix A 253 g + B 126.5 g = 330 ml total (379.5 g)
craftcalc.io — free maker calculators with the arithmetic shown
How much epoxy resin does a mould need?
Resin is measured by volume and mixed by ratio, so the calculation has two steps. First, the mould volume: a rectangular mould is length × width × depth; a cylinder is π × r² × depth; a full sphere is 4/3 × π × r³ and a dome is half that. In centimetres, the answer comes out in cm³, which equals millilitres. Multiply by the number of cavities and add roughly 10% — resin clings to mixing cups, sticks and gloves, and you can never quite get the last of it out.
Second, the split. Epoxy comes in ratios like 1:1, 2:1, 3:1 or 100:33, and the label always says whether that ratio is by weight or by volume — the two are not interchangeable because part B is usually less dense than part A. Mixing a by-volume resin on a scale (or vice versa) is the most common cause of tacky, never-curing pours. This calculator splits the total both ways; pick the one your bottle says.
Worked example: a 150 × 100 × 20 mm tray
A rectangular tray mould, 15 × 10 × 2 cm, in a 2:1 by-weight epoxy. Mould volume: 15 × 10 × 2 = 300 ml. With 10% mixing loss: 330 ml, which at 1.15 g/ml is 380 g of mixed resin. Split 2:1 by weight: part A = 380 × 2/3 = 253 g, part B = 127 g. Weigh A into a clean cup, add B, scrape the sides and bottom while stirring for a full two minutes, then pour from a second clean cup so the un-mixed film from the first cup stays behind.
The same mould as a dome (say a 60 mm hemisphere for a paperweight): 2/3 × π × 3³ = 56.5 ml per dome — about a fifth of the tray. Small pours like that are where the 10% allowance matters most: 5 ml lost on a 56 ml pour is nearly 10% by itself.
Frequently asked questions
Why did my resin stay sticky after curing?
Almost always a ratio or mixing error: the wrong ratio for the product (1:1 and 2:1 are not interchangeable), measuring a by-volume resin by weight, or under-mixing. Scrape the sides and bottom of the cup while mixing for two full minutes, and measure exactly the way the label specifies.
How do I find my mould's volume if it is an odd shape?
Fill it with water to the pour line and measure the water — a kitchen scale reading grams reads millilitres directly. Use the 'I know the volume' shape and enter it. This beats geometry for any mould with curves or texture.
Does temperature change how much resin I need?
Not the amount, but everything else: cold resin is thick and traps bubbles; warm resin cures faster with less pot life. Work at 21–24°C / 70–75°F, and warm the bottles in a water bath (not hot) if the resin is cold and cloudy.
How much extra resin should I mix?
10% is the working allowance here — cups, sticks and the film you cannot pour out. For deep pours in stages, add a little more per layer since each stage wastes its own cup. Never mix 'a bit extra' into the mould itself: overfilled resin cures into a lens you have to sand off.
Can I convert a by-weight ratio to volume?
Only with each part's density from the technical data sheet (A ÷ its density = volume). Guessing the conversion is how pours fail — if the label says by weight, use a $15 digital scale rather than measuring cups.
Formulas and standard tables are sourced in the methodology. Results are estimates — measure twice, cut once.