Volume first, then the split
Every resin question is the same two steps. Work out how much space you are filling, then divide that between the two components.
The first step is easier in metric than most people expect, because 1 cm³ is exactly 1 millilitre. Work in centimetres and the volume you calculate is already the number of millilitres you need.
A 40 × 30 cm tray poured 1 cm deep: 40 × 30 × 1 = 1,200 cm³ = 1,200 ml = 1.2 litres. In imperial, a 16 × 12 inch board poured a quarter inch deep is 16 × 12 × 0.25 = 48 cubic inches, which is 787 ml — and there the conversion is not free, which is why the calculator does it for you.
The mix ratio trap
This is the section worth reading even if you never touch the calculator.
A two-part epoxy is sold with a mix ratio — 1:1, 2:1, 3:1, 100:45. That ratio is published either by volume or by weight, and the two are the same number only when the resin and the hardener happen to have the same density. They usually do not.
Take a system that is 1:1 by volume, with resin at 1.15 g/cm³ and hardener at 0.96 g/cm³. Pour equal volumes and you have equal volumes — correct. Weigh equal masses and you have more hardener than resin by volume, because the hardener is lighter. Expressed properly, that 1:1-by-volume system is 100:83.48 by weight.
Real products bear this out. ArtResin and MAS Table Top Pro are both 1:1 by volume; their published weight ratios are 100:84 and 100:83. Mix either at 1:1 on a scale and the chemistry is off by nearly a fifth on one component, which shows up as a surface that stays tacky and never cures.
So: read the data sheet, note which basis the ratio uses, and measure that way. This calculator asks you which, splits on that basis, and converts between the two only when you have given it both densities. Where it cannot convert honestly, it says so instead of producing a number.
Measuring awkward shapes
Rectangles, circles, rings and triangles are built into the calculator, and a cylinder is just a circle with depth. What follows is for everything else.
A river table. Measure the void, not the table. Length is easy; width is the awkward part, because live edges are not parallel. Measuring the widest and narrowest points and taking the average is accurate enough at the tolerance a pour needs. Treat the result as a rectangle.
Anything genuinely irregular. Fill the mould with water, then pour that water into a measuring jug. That is your volume, exactly, with no geometry involved at all. Dry the mould thoroughly afterwards — water and uncured epoxy do not get on. Use the "I already know the volume" option and enter the reading.
A mould with something in it. Wood, flowers, dried botanicals, pigment chips — anything already in the mould is space the resin does not need to fill. Estimate the share by eye for small inclusions; for a substantial piece of wood, submerge it in a jug of water and read the rise, which gives its volume directly. Enter that as a percentage of the mould.
Depth is a safety question, not a volume one
The arithmetic does not care how deep the pour is. The chemistry very much does.
Epoxy cures exothermically — the reaction gives off heat. In a thin layer that heat escapes into the air and nothing happens. In a thick mass it cannot escape, so the temperature rises, and a warmer resin reacts faster, which produces more heat. A pour that is too thick for its product can smoke, crack internally, yellow, or in the worst case scorch the mould.
The maximum safe thickness per layer is a property of the product, not of the shape. Deep-pour casting resins are formulated for thicknesses measured in inches; table-top and coating resins are usually limited to a few millimetres per layer and must be built up in coats. The number is on the data sheet.
The calculator flags anything over about an inch, not because an inch is a universal limit but because that is where the question stops being academic.
Mixing extra
Mix a little more than the calculation says. Resin is lost to:
- the film left clinging to the mixing cup and the stir stick;
- absorption into bare wood, which can be surprising on an open-grained species;
- whatever escapes a mould that turns out not to be perfectly sealed;
- the pour you do not quite finish level.
There is a percentage field for this and the calculator does not suggest a figure, because a percentage that is sensible on a large pour is meaningless on three coasters. Think about it in absolute terms instead: a mixing cup and stick hold roughly the same amount of residue whether you mixed 200 ml or two litres.
Running out mid-pour is much worse than having some left over. A second batch mixed and poured onto a partly-cured first batch leaves a visible line, and on a river table it can leave a structural one.
Volume, weight and density
To turn millilitres into grams you need a density, and this calculator will not invent one.
Most epoxy casting systems land between about 1.0 and 1.2 g/cm³, so a litre weighs somewhere between 1.0 and 1.2 kg. That band is wide enough that taking the middle of it introduces a real error on a large pour — on ten litres, the difference between the ends of the range is two kilograms of material.
The figure is printed on the technical data sheet. Enter it and the calculator gives weights as well as volumes, and can convert your ratio between the two bases. Leave it out and the volume answer is still complete; the weight answer is simply withheld rather than guessed.
What a kit actually covers
Resin is sold by volume and used by area, and the bridge between them is the pour depth. In metric the arithmetic is unusually friendly:
One litre covers one square metre at one millimetre deep.
That falls straight out of the units — a litre is 0.001 m³, and 0.001 m³ spread over 1 m² is 1 mm — and everything else scales from it. At 2 mm a litre covers half a square metre; at 3 mm, a third; at 5 mm, a fifth.
| Pour depth | 1 litre covers | A 5 litre kit covers |
|---|---|---|
| 1 mm | 1.00 m² | 5.0 m² |
| 2 mm | 0.50 m² | 2.5 m² |
| 3 mm | 0.33 m² | 1.7 m² |
| 5 mm | 0.20 m² | 1.0 m² |
| 10 mm | 0.10 m² | 0.5 m² |
In imperial the equivalent is that a US gallon at an eighth of an inch covers about 12.8 square feet. It is a less memorable number because neither unit is a power of ten of the other, which is a good argument for measuring a pour in millimetres even if everything else on the bench is in inches.
A note on coating resins specifically: the depth that matters for coverage is the finished film, not the puddle you pour. A self-levelling table-top resin spreads to its own equilibrium thickness, and the manufacturer's coverage figure is for that thickness rather than for whatever you poured. If the published coverage and your own calculation disagree by a wide margin, that assumption is usually the reason — check which depth their figure was quoted at before deciding one of you is wrong.
Frequently asked questions
How much resin do I need?
Work out the volume of the space you are filling, in cubic centimetres or cubic inches, then convert. A 40 × 30 cm tray poured 1 cm deep is 40 × 30 × 1 = 1,200 cm³, which is 1.2 litres — and because 1 cm³ is exactly 1 ml, that conversion is free. Add a little for what stays in the mixing cup, and subtract for anything already in the mould.
Is a 2:1 mix ratio by volume or by weight?
Whichever the manufacturer says, and you must not assume. The two are only the same when the resin and hardener have the same density, which they usually do not. Mixing a volume ratio on a scale is the classic way to end up with a tacky surface that never fully cures. The technical data sheet states which basis its ratio uses — often both, with different numbers.
Can I convert a volume ratio into a weight ratio?
Only if you know both densities. Multiply each part of the volume ratio by its own component density and renormalise. A 1:1 volume system with resin at 1.15 g/cm³ and hardener at 0.96 g/cm³ is 100:83.48 by weight, not 1:1. The calculator will do this conversion when you give it both densities and will refuse when you do not, because a guessed mix ratio is exactly the failure this page exists to prevent.
How much resin for a river table?
Measure the void, not the table. Work out the length, the average width of the gap and the depth, and treat it as a rectangle; where the edges are live and irregular, measuring the widest and narrowest points and averaging them is close enough at the accuracy a pour needs. Then add a margin — a river table is the worst place to run out mid-pour.
How do I allow for wood, flowers or other inclusions?
Use the displacement field. Anything already in the mould takes up space the resin does not have to fill, so a mould that is a quarter full of wood needs a quarter less resin. Estimating that share by eye is fine for small inclusions; for something substantial, the water displacement trick — submerge it in a measuring jug and read the rise — gives a real number.
How thick can I pour in one go?
That depends entirely on the product and it is a safety question rather than an arithmetic one. Epoxy cures exothermically: a deep pour generates heat, the heat speeds the reaction, and a mass that is too thick can smoke, crack, yellow or in bad cases burn. Casting resins sold for deep pours state a maximum thickness per layer; table-top and coating resins are usually limited to a few millimetres. The calculator flags a pour over about an inch so the question gets asked.
How much extra should I mix for waste?
Something, and the right figure depends on your setup rather than on a rule. Resin left clinging to the cup and the stir stick, resin absorbed into bare wood, and resin lost to a leaking mould all count. The calculator has a percentage field so you can apply your own figure; it does not suggest one, because 5% on a large pour and 5% on three coasters are wildly different amounts of actual liquid.
What does a litre of resin weigh?
Whatever its density says, and the calculator will not guess. Most epoxy casting systems land somewhere between 1.0 and 1.2 kg per litre, which is a wide enough band that using the middle of it introduces a real error on a large pour. The figure is on the technical data sheet — enter it and the calculator converts; leave it out and you still get the full volume answer.
Does the calculator handle round moulds and rings?
Yes. Rectangles, circles, rings with a hole through the middle, triangles and cubes are all built in, and there is a free-volume option if you have measured the capacity another way — by filling the mould with water and weighing or measuring it, which is the most reliable method of all for an irregular shape.
Why does the calculator give ounces as well as millilitres?
Because resin is sold both ways and the kits rarely match the mould. The fluid ounces shown are US fluid ounces, where a US gallon is 3.785411784 litres exactly. Weights in ounces are avoirdupois ounces of 28.349523125 g. Both conversions are exact by definition rather than rounded.
Are my measurements sent anywhere?
No. Every calculation runs in JavaScript in your own browser. Nothing you type is transmitted or stored.