How much resin do I need for my mould?

Measure the inside of the mould and the page does the rest. A block 20 x 10 x 2 cm is 400 cm3, and a cubic centimetre is a millilitre, so that is 400 mL of casting. At the 1.10 g/cm3 that mixed casting epoxy is usually published at, 400 mL weighs 440.0 g, and a ten percent allowance for what stays in the cup and on the stick makes 484.0 g of mix to weigh out. Round moulds, a volume you already know from the mould's label and a weight you have already decided on are all starting points too.

Is a 2:1 resin ratio by weight or by volume?

Both are printed on real bottles, which is why this page asks instead of guessing. Take the 484.0 g pour above. Read as 2:1 by weight it is 322.7 g of resin and 161.3 g of hardener on a scale. Read as 2:1 by volume it is a 440.0 mL mix split into 293.3 mL and 146.7 mL in measuring cups. Those are different mixes, and the page answers in the basis you picked and leaves it there.

Why will it not convert a volume ratio into weights?

Because that conversion needs the density of the resin on its own and of the hardener on its own, and a bottle prints the mixed figure at most. Resin and hardener are different chemistries with different densities, so turning 293.3 mL of part A into grams with the mixed density would be a guess dressed up as arithmetic. When you enter the mixed density, this page uses it for the one job it is good for: turning the whole pour between millilitres and grams.

How do I mix 300 g of a 2:1 epoxy?

Start from the weight instead of the mould: enter 300 g with no waste allowance and a 2:1 ratio by weight gives 200 g of resin and 100 g of hardener. No density is needed for that answer, and the page says so rather than demanding one: the volume cell reads Needs a density until you enter one, and everything else still works.

What density should I use for epoxy resin?

The figure on the datasheet for the resin in front of you. The buttons hold the values manufacturers commonly publish for a mixed resin, 1.10 for epoxy, 1.20 for polyester and 1.05 for urethane, and they are a starting point rather than a measurement of your bottle. Pigments, fillers and a different hardener all move it. Change the figure and every weight on the page moves with it, which is the point of typing your own.

How much extra should I mix for waste?

Ten percent is the usual allowance and it is what the page opens with: 440.0 g of casting becomes 484.0 g of mix. It covers what clings to the mixing cup, the stick and the sides of the pot, so a wide shallow mould poured from a big cup wastes more than a deep narrow one. Set it to zero and the mix is the casting exactly.

Does it work in inches, and with round moulds?

Yes to both, and mould and mold are the same thing here. A 4 x 4 x 0.5 inch tray is 131.1 mL, because an inch is exactly 2.54 cm. A round mould is worked out as pi times the radius squared times the depth, so 7 cm across and 1 cm deep is 38.5 mL, and six of them is 230.9 mL. Pi has no exact decimal, so that one figure is a numerical value carried at full precision and rounded only where it is printed, while a block is plain multiplication.

How deep can I pour, and how long will it take to cure?

This page does not answer either question, and no calculator should. Pour depth is limited by the heat a resin gives off as it sets, and both that and the cure schedule are properties of the exact resin and hardener you bought, printed on their technical datasheet. What you get here is how much to mix and how to split it.

Resin Calculator

Two numbers decide a pour and most pages answer only the first. The volume of the mould is arithmetic: 20 x 10 x 2 cm is 400 cm3, which is 400 mL, and a ten percent allowance for what stays in the cup makes it 440 mL of mix. The second number is the mix ratio on the bottle, and that is where the pages stop: 2:1 means one thing in grams and another in millilitres, and neither converts into the other without knowing what resin alone and hardener alone each weigh. So the basis is a control here. At 1.10 g/cm3 that pour is 484.0 g, and 2:1 by weight splits it into 322.7 g of resin and 161.3 g of hardener, printed to the same decimals so the two visibly add back to the total. Ask for the same 2:1 by volume and the answer is 293.3 mL and 146.7 mL, in millilitres, never quietly restated as a weight.

Block 20 x 10 x 2 cmDensity 1.10 g/cm3Ratio 2:1 by weightWaste 10%

mould       20 x 10 x 2 cm  =  400 cm3  =  400 mL
density     1.10 g/cm3
mass        440.0 g
waste 10%   484.0 g total

ratio       2 : 1  by weight
resin A     322.7 g
hardener B  161.3 g
            -------
            484.0 g

Volume: 400 mL
Total: 484.0 g
Resin A: 322.7 g
Hardener B: 161.3 g

volume = 20 x 10 x 2 cm = 400 cm3; mass = 400 mL x 1.10 = 440.0 g; waste = +10% = 484.0 g; split 2 : 1 by weight = 322.7 g + 161.3 g

A block mould is length x width x depth: 20 x 10 x 2 cm is 400 cm3, and a cubic centimetre is a millilitre.
Volume becomes weight through the density you entered, 1.10 g/cm3: 400 mL x 1.10 is 440.0 g. The figure on your resin's datasheet is the one to use.
Waste adds 10% on top for what stays in the cup and on the stick: 440.0 g becomes 484.0 g.
The ratio is 2 : 1 by weight, so 484.0 g splits into 322.7 g and 161.3 g, and the two add back to 484.0 g.
A ratio is not converted between weight and volume here: that needs the density of resin and hardener separately, and a bottle prints the mixed figure at most.

The mould

Measure the inside of the mould, or start from a volume or a weight you already know. Every route ends at the same split.

Start from

LengthWidthDepthMeasured in

How many of themWaste allowance, percent

The waste allowance is the resin that stays in the cup and on the stick rather than in the casting. Ten percent is the usual figure, and a deep narrow mould wastes less than a wide shallow one.

The density

This is the bridge from a volume to a weight, and there is no single right figure: the buttons hold the values manufacturers usually publish for a mixed resin, and the one on your own datasheet beats all of them. A ratio that stays in the unit you started in does not need it at all.

Density, g/cm3Figures manufacturers publish

The mix ratio

Type the ratio the way the bottle prints it, then say which basis it is in. A 2:1 by weight and a 2:1 by volume are different splits of the same mix, and neither is converted into the other here.

Ratio, resin to hardenerThat ratio is

This page works out quantities. It does not work out cure time, exotherm or how deep one pour can go: those come from the datasheet for the resin in your hand.
The volume is the space inside the mould. Anything you set into the casting takes up part of that space, so take it off yourself.
Nothing here is a statement about what a finished casting can hold.

Worked examples

Each one runs through the same function as your own numbers. Load one to see it in the fields.

2:1 by weight, 300 g totalthe manufacturer's ratio on a known mix
200 g resin, 100 g hardener
20 x 10 x 2 cm at 1.10 g/cm3before waste
400 mL, 440.0 g
Ratio given by volumeno component densities
293.3 mL and 146.7 mL, never converted to weight

Quantities, not a cure schedule. Exact arithmetic on the mould dimensions, density and ratio you enter, with the parts always adding back to the total. The ratio stays in the basis the manufacturer printed, by weight or by volume, because converting between the two needs each component's own density. It says nothing about cure time, exotherm or what a casting can hold.

Common questions

How much resin do I need for my mould?: Measure the inside of the mould and the page does the rest. A block 20 x 10 x 2 cm is 400 cm3, and a cubic centimetre is a millilitre, so that is 400 mL of casting. At the 1.10 g/cm3 that mixed casting epoxy is usually published at, 400 mL weighs 440.0 g, and a ten percent allowance for what stays in the cup and on the stick makes 484.0 g of mix to weigh out. Round moulds, a volume you already know from the mould's label and a weight you have already decided on are all starting points too.
Is a 2:1 resin ratio by weight or by volume?: Both are printed on real bottles, which is why this page asks instead of guessing. Take the 484.0 g pour above. Read as 2:1 by weight it is 322.7 g of resin and 161.3 g of hardener on a scale. Read as 2:1 by volume it is a 440.0 mL mix split into 293.3 mL and 146.7 mL in measuring cups. Those are different mixes, and the page answers in the basis you picked and leaves it there.
Why will it not convert a volume ratio into weights?: Because that conversion needs the density of the resin on its own and of the hardener on its own, and a bottle prints the mixed figure at most. Resin and hardener are different chemistries with different densities, so turning 293.3 mL of part A into grams with the mixed density would be a guess dressed up as arithmetic. When you enter the mixed density, this page uses it for the one job it is good for: turning the whole pour between millilitres and grams.
How do I mix 300 g of a 2:1 epoxy?: Start from the weight instead of the mould: enter 300 g with no waste allowance and a 2:1 ratio by weight gives 200 g of resin and 100 g of hardener. No density is needed for that answer, and the page says so rather than demanding one: the volume cell reads Needs a density until you enter one, and everything else still works.
What density should I use for epoxy resin?: The figure on the datasheet for the resin in front of you. The buttons hold the values manufacturers commonly publish for a mixed resin, 1.10 for epoxy, 1.20 for polyester and 1.05 for urethane, and they are a starting point rather than a measurement of your bottle. Pigments, fillers and a different hardener all move it. Change the figure and every weight on the page moves with it, which is the point of typing your own.
How much extra should I mix for waste?: Ten percent is the usual allowance and it is what the page opens with: 440.0 g of casting becomes 484.0 g of mix. It covers what clings to the mixing cup, the stick and the sides of the pot, so a wide shallow mould poured from a big cup wastes more than a deep narrow one. Set it to zero and the mix is the casting exactly.
Does it work in inches, and with round moulds?: Yes to both, and mould and mold are the same thing here. A 4 x 4 x 0.5 inch tray is 131.1 mL, because an inch is exactly 2.54 cm. A round mould is worked out as pi times the radius squared times the depth, so 7 cm across and 1 cm deep is 38.5 mL, and six of them is 230.9 mL. Pi has no exact decimal, so that one figure is a numerical value carried at full precision and rounded only where it is printed, while a block is plain multiplication.
How deep can I pour, and how long will it take to cure?: This page does not answer either question, and no calculator should. Pour depth is limited by the heat a resin gives off as it sets, and both that and the cure schedule are properties of the exact resin and hardener you bought, printed on their technical datasheet. What you get here is how much to mix and how to split it.

Exact arithmetic on the mould dimensions, density and ratio you enter, with the parts always adding back to the total. The ratio stays in the basis the manufacturer printed, by weight or by volume, because converting between the two needs each component's own density. It says nothing about cure time, exotherm or what a casting can hold.