How much epoxy resin and hardener you need for a pour

How the amounts of resin and hardener are worked out

You multiply the volume to fill by the density of the mix, then split that weight in the ratio on the data sheet. The calculator does it in four steps.

1. Volume. Length × width × thickness for a rectangle, 3.14 ÷ 4 × diameter² × thickness for a disc. For a mold with an awkward shape, fill it with water and pour the water into a measuring cup: that is the volume to enter under Known volume.
2. Density of the mix. Data sheets give the density of part A and part B. Some US sheets give it in pounds per gallon: divide by 8.345, the weight of a gallon of water, to get the specific gravity. With a ratio a:b by weight the mix comes to (a + b) ÷ (a ÷ density of A + b ÷ density of B); with a ratio by volume, (a × density of A + b × density of B) ÷ (a + b).
3. A and B. The mix, waste included, is split in the ratio: with 2:1 by volume, resin is 2 parts out of 3. Each part is shown by weight and by volume, because A and B do not weigh the same. TotalBoat Fathom is 2:1 by volume but 100:44 by weight.
4. Pours and kits. The thickness divided by the maximum depth per pour on the data sheet, rounded up, gives the pours. The mix divided by the contents of one kit (A + B), rounded up, gives the kits.

Waste starts at 10%, our own estimate that you can change in the advanced options. It covers what stays in the mixing cups, on the stir stick and on the mold edges; TotalBoat also notes that its coverage figures do not account for material wasted on tools or left in the bottle. The densities of A and B are in the advanced options too.

Worked example: a 48 × 8 in river, 1 1/2 in deep

These are the values the calculator opens with in US units, using the TotalBoat Fathom data sheet: 2:1 by volume, resin 9.22 lb/gal and hardener 8.2 lb/gal (specific gravity 1.10 and 0.98), up to 2 in per pour for river tables, 1.5 gallon kits.

• Volume: 48 × 8 × 1.5 = 576 cubic inches, which is 2.49 gal
• Mix: specific gravity 1.06; with 10% waste, 2.74 gal or 24.26 lb
• Part A: 1.83 gal, which weighs 16.79 lb
• Part B: 0.91 gal, which weighs 7.48 lb
• Pours: 1 1/2 in is under the 2 in limit, so one pour
• Kits: 2.74 ÷ 1.5 = 1.83, so 2 kits of 1.5 gal, with about 33 fl oz left over

If you weigh instead of measuring by volume, the same ratio becomes 100:44.5 by weight; the data sheet rounds it to 100:44.

In the UK and with metric units the calculator opens on a pour 40 × 25 cm and 2 cm deep with Yachticon Brilliant Shine (100:45 by weight, A 1.12 and B 1.00 g/cm³, 10 mm per pour, 1.5 L kits). That is 2 L, or 2.38 kg of mix with 10% waste: 1.64 kg of A (1.46 L) and 737 g of B (737 ml), in 2 pours of 10 mm and 2 kits of 1.5 L, with 800 ml left over.

Mix ratios by weight and by volume

The ratio by weight and the ratio by volume of the same resin are different. The table compares four data sheets; the last two columns are worked out from the densities of A and B given on each sheet.

Resin	Data sheet	Other way	1 L of mix
TotalBoat Fathom	2:1 by volume	100:44.5 by weight	733 g A + 327 g B
TotalBoat ThickSet	1:1 by volume	100:89.6 by weight	575 g A + 515 g B
Brilliant Shine	100:45 by weight	100:50.4 by volume	745 g A + 335 g B
ProCa EP1-26	100:50 by weight	100:57.9 by volume	697 g A + 348 g B

The sheets that give both ratios agree with the calculation: Fathom says 100:44 by weight, ThickSet 1.11:1 by weight (100:90), Brilliant Shine 100:50 by volume. A quart is 0.95 L, so a quart of mix takes about 5% less than the last column.

Kit sizes and maximum depth per pour

Kits come with A and B already in the right ratio, sold by volume or by weight. Fathom comes in 1.5, 3 and 6 gallon kits; ThickSet from an 8 oz kit (two 4 oz bottles) through pint and quart kits up to a gallon. In Europe Brilliant Shine comes in 0.75 L (0.5 L of resin and 0.25 L of hardener), 1.5 L and 7.5 L. In the Kit sold field pick By volume or By weight as on the label, then enter the total contents, resin plus hardener.

The maximum depth per pour tells you which resin to buy. Brilliant Shine, a coating resin, pours 3 to 10 mm thick; ThickSet up to 2 in in molds but 1/2 in for river tables; Clearcast Deep Pour 50 mm in one go; Fathom up to 2 in for river tables and 3 in for smaller castings. The thicker the pour, the hotter the reaction: TotalBoat warns that going past the stated depth can cause yellowing, shrinkage or cracking. When the calculator counts more than one pour, mix one pour at a time and wait as long as the data sheet says before the next one.

Common mistakes

• Using a weight ratio as a volume ratio. Clearcast warns that its 100:45 by weight resin must not be mixed by volume. With the Brilliant Shine densities, 100:45 measured in cups comes out at 100:40.2 by weight, 11% less hardener than needed.
• Taking the density of the cured resin. Some sheets give the density of the finished piece, 1.10 g/cm³ for Brilliant Shine and 1.11 for Fathom, while the liquid mix worked out from A and B comes to 1.08 and 1.06. For kits sold by weight you need the liquid figure, which the calculator works out for you.
• Pouring the whole depth at once. The maximum depth applies to each pour, not to the finished piece: a 3 in slab with a 2 in resin needs two pours.
• Using a deep pour resin for thin pieces. TotalBoat advises against Fathom for pours under 1 in deep and asks for at least half a gallon of mix; coasters and jewelry call for a resin made for thin layers.
• Forgetting shrinkage. Fathom lists 4.7% volumetric shrinkage: the cured piece takes up a little less room than the liquid you poured, and the 10% waste covers that too.