How to calculate epoxy: the volume formula, with examples

The one formula

Epoxy is sold by volume, so you need a volume. For anything box-shaped, that’s length × width × depth. For anything round, it’s π × radius² × depth. A coating is just a very thin box: the area you’re coating multiplied by the thickness of the coat.

box or coating:  volume = length × width × depth
circle:         volume = 3.1416 × (diameter ÷ 2)² × depth
then add waste: total = volume × (1 + waste allowance)

The only rule is to use the same unit for every measurement. Measure everything in inches and you get cubic inches; measure everything in centimeters and you get cubic centimeters, which are the same as milliliters. Mixing feet and inches in one multiplication is the most common arithmetic slip.

Converting to the units on the kit

Once you have cubic inches or milliliters, convert to whatever the kit is sold in. These are the conversions you’ll use most:

Coverage per gallon at common thicknesses

For coatings — a seal coat, or a flood coat (the clear, self-leveling top layer poured over a table, usually about 1/8 in thick) — it’s often quicker to think in coverage: at a given thickness, a gallon always covers the same area. Divide 231 by the thickness in inches, then by 144, and you have square feet per gallon. The short table below covers the common coats; the epoxy coverage chart has every thickness from 1/32 in to 1 in, metric, and floor coatings in mils.

A seal coat is thinner than any row here suggests: it is brushed or scraped into bare wood, and a seal coat ≈ 1–2 fl oz per sq ft (0.3–0.6 L/m²), more on spalted or end grain. The tabletop calculator lets you pick the rate for your wood.

Three worked examples

1. A 6 ft river table

Worked in full on the 6 ft river table example page, with other widths and depths.

The river runs 72 in, averages 8 in wide, and the slabs are 1.5 in thick. Most people pour about 1/16 in proud of the wood so the top can be flattened, so the depth is 1.5625 in. 72 × 8 × 1.5625 = 900 cubic inches. 900 ÷ 231 = 3.9 gallons. With a 10% allowance: 3.9 × 1.1 = 4.29 gallons of mixed epoxy. For a 1:1 product that’s about 2.14 gallons each of resin and hardener; a 2:1 split is worked below.

2. A flood coat on a 60 × 30 in desk

60 × 30 = 1,800 sq in. At 1/8 in: 1,800 × 0.125 = 225 cubic inches, or 0.97 gallons. Flood coats run off the edges, so count the edge: 180 in (15 ft) of it drips about 1.25 fl oz per foot, or 18.8 fl oz, and the sides hold a thin film of about 4.7 fl oz (180 × 1.5 in tall × 1/32 in). Add 5% for the bucket: about 1.21 gallons. A 1-gallon kit is not enough. The 60 × 30 in desk example adds the seal coat that bare wood needs first.

3. Four resin coasters

Each coaster is 4 in across and 3/8 in deep: 3.1416 × 2 × 2 × 0.375 = 4.71 cubic inches, or 2.61 fl oz. Four of them is 10.4 fl oz; with 10% extra, about 11.5 fl oz. The set of 4 coasters example shows the same pour with other depths and counts.

How much extra to buy

Every project loses some epoxy — to the sides of the mixing bucket, the stir stick, drips off an edge, a small leak in a form, or wood that soaks it up. The right allowance depends on the job:

Where estimates go wrong

• Measuring the whole slab instead of the river. For a river table, only the gap between the slabs gets filled.
• One width measurement on a live edge. Measure the gap every foot or so and average it.
• Forgetting the flood coat. A river table finished with epoxy needs a deep pour and a coating over the whole top — two different products, two separate calculations.
• Weight vs volume. Epoxy is heavier than water, and resin and hardener have different densities. If your product gives a ratio by weight, don’t measure it by volume.
• Depth limits. Having enough epoxy doesn’t mean you can pour it all at once. Check the maximum pour depth and plan lifts.

Split into Part A and Part B

Kits are sold as two parts mixed at a set ratio. To split a total by volume, divide the total by the sum of the ratio numbers and multiply by each side. For the 4.29 gallons of the river table above, with a 2:1 epoxy: 4.29 ÷ 3 = 1.43, so 2.86 gallons of Part A and 1.43 gallons of Part B.

Part A = total × A ÷ (A + B)
Part B = total × B ÷ (A + B)

If you measure by weight, don’t use the volume ratio on a scale — convert it first using each part’s weight per ml (the “specific gravity” on the data sheet).

From volume to kits and cost

Finally, round up to whole kits. Compare kit sizes — two 1-gallon kits sometimes cost more than one 2-gallon kit, but sometimes a smaller kit leaves less waste. And note the depth or thickness the product is designed for: deep-pour and tabletop epoxies are not interchangeable (see deep pour vs tabletop epoxy).