Coverage is one multiplication and two conversions
Every epoxy quantity question reduces to volume, and volume is area times depth. The complications people run into are not mathematical: they are that area arrives in square feet, thickness arrives in sixteenths of an inch or in millimetres, and the product is sold in gallons or litres. Get those three units into one system and the answer falls out in a line.
The imperial version uses two exact constants. There are 144 square inches in a square foot and exactly 231 cubic inches in a US liquid gallon. Multiply your area in square inches by your thickness in inches to get cubic inches, then divide by 231. The metric version needs no constants at all: one litre spread over one square metre is exactly one millimetre thick, so litres = square metres × millimetres. That identity is worth memorising, because it turns a whole class of quoting questions into mental arithmetic.
The reason a calculator still earns its place is everything hanging off that core: voids that are not part of the flat surface, a waste allowance for what stays in the bucket, the maximum depth your particular resin can be poured in one go, and the step from "gallons required" to "kits I can actually buy". Those four are where the money and the failed pours live.
What each term is doing
Area × thickness gives the film. Measure inside the dams, not the outside of the slab, and use the wetted area rather than the visual one — a live edge that the resin runs over adds area you did not draw.
Void volume is added separately because it has no relationship to the surface. A river-table channel, a bark inclusion, a knot or a split does not scale with the tabletop's thickness, and the only reliable way to measure it is to seal the underside and fill the cavity with water or dry rice, then pour that into a measuring jug. Guessing a channel's volume from its outline is the single most common cause of a river table running short mid-pour.
The waste allowance covers what never reaches the workpiece: film clinging to the mixing bucket and stir stick, resin that runs off the edge before it gels, and the sacrificial spread on a squeegee. Ten per cent is a reasonable default for a self-levelling flood coat. Raise it for a small pour, where the bucket film is a larger fraction of the total, and for any pour with unretained edges.
Maximum lift depth is not a convenience figure. Epoxy cures exothermically — the reaction generates heat, and heat accelerates the reaction. In a thin film that heat escapes through the surface. In a deep mass it accumulates, and a table-top formulation poured too deep can reach temperatures that yellow it, craze it, or in extreme cases smoke. Deep-pour casting resins are formulated with slower systems specifically so the heat has time to leave. The number to enter is the one on your product's technical data sheet, and dividing your target thickness by it, rounded up, tells you how many separate pours the job takes.
Worked example: a 96 × 25 in bar top at 1/8 in
You are flooding a bar top that measures 96 in by 25 in with a 1/8 in flood coat, using a table-top resin with a 1/4 in maximum lift, allowing 10% waste, buying 1-gallon mixed kits at $100 a gallon.
- Area. 96 × 25 = 2,400 in², which is 2,400 ÷ 144 = 16.67 ft².
- Film volume. 2,400 in² × 0.125 in = 300 in³.
- Waste. 300 × 1.10 = 330 in³.
- Gallons. 330 ÷ 231 = 1.429 gal. In litres: 330 × 0.016387 = 5.41 L.
- Kits. 1.429 ÷ 1 gal per kit, rounded up = 2 kits, costing $200 — and leaving 0.57 gal in stock.
- Lifts. 0.125 in ÷ 0.25 in = 0.5, rounded up = 1 pour. The flood coat goes on in one go.
- Cross-check the coverage. 231 ÷ (144 × 0.125) = 12.83 ft² per gallon. Your 16.67 ft² ÷ 12.83 = 1.30 gal before waste, and 1.30 × 1.10 = 1.43 gal. ✓
Now change one thing: suppose the same bar top has a 60 in long river channel that holds 210 in³ of water when you test-fill it, and you switch to a deep-pour resin rated to 2 in per lift. The total becomes (300 + 210) × 1.10 = 561 in³ = 2.43 gal, three kits, still one lift because the channel is 1.5 in deep. The channel is 41% of the material and none of it is visible in the surface area.
How to read the result
Round up to whole kits, and expect leftovers. Epoxy kits come in fixed sizes, so a job needing 1.43 gal buys 2 gal. Do not be tempted to stretch the resin by pouring thinner than planned: as a working rule, below about 1/16 in a flood coat stops self-levelling reliably over a wide surface and starts showing every trowel mark and every low spot in the substrate.
Check the lift count before you check anything else. If it is greater than one, your job is no longer a single afternoon. Each lift has to go on inside the recoat window on the data sheet — the period during which the previous lift is still chemically able to bond to the next. Miss it and the surface must be scuff-sanded and cleaned before the next pour, which is extra work and a possible visible line.
Compare the coverage-per-gallon figure with whatever the supplier's marketing says. At 1/8 in a gallon covers 12.83 ft²; at 1/16 in it covers 25.7 ft²; at 1/32 in, a seal coat, it covers 51.3 ft². Any claim outside those numbers is either quoting a different thickness or quoting the unmixed resin volume rather than the mixed volume, and it is always worth asking which.
Two thicknesses matter for most projects and both should be budgeted. A seal coat at roughly 1/32 in is brushed on first to lock down porous end grain and stop bubbles rising out of the wood into the finish pour. A flood coat at 1/8 in is the one that produces the glass-like surface. Run this calculator twice, once at each thickness, and add the results.
Epoxy coverage per gallon and per litre by thickness
| Thickness | mm | ft² per US gallon | m² per litre | Typical use |
|---|---|---|---|---|
| 1/32 in | 0.79 | 51.33 | 1.26 | Seal coat on bare wood |
| 1/16 in | 1.59 | 25.67 | 0.63 | Thin art panel, second coat |
| 3/32 in | 2.38 | 17.11 | 0.42 | Light flood coat |
| 1/8 in | 3.18 | 12.83 | 0.31 | Standard flood coat, bar top |
| 3/16 in | 4.76 | 8.56 | 0.21 | Heavy flood coat |
| 1/4 in | 6.35 | 6.42 | 0.16 | Top of the single-lift range for most table-top resins |
| 1/2 in | 12.70 | 3.21 | 0.08 | Deep-pour formulations only |
| 1 in | 25.40 | 1.60 | 0.04 | Casting, river channels |
The "typical use" column describes common practice, not a specification. Your resin's technical data sheet governs the maximum depth you may pour in one lift.
Where epoxy estimates go wrong
- Measuring the outside of the slab. Live edges, overhangs and dams change the wetted area. Measure what the resin will actually touch.
- Estimating a river channel from its outline. Channel walls are rarely vertical and rarely uniform. Seal the underside and fill it with water or rice, then measure that volume.
- Confusing resin volume with mixed volume. A kit sold as "one gallon" may mean one gallon of resin plus a separate hardener, giving 1.33 gal mixed at a 3:1 ratio. Enter the mixed volume in the kit-size field.
- Ignoring absorption into bare wood. Porous end grain, spalted timber and open-pored species drink the first coat. Seal first, then flood — otherwise the flood coat thins unpredictably and gasses out.
- Pouring deeper than the data sheet allows. This is a heat problem, not a patience problem. The calculator's lift count exists so you plan multiple pours rather than discovering the limit the hard way.
- Forgetting that self-levelling has limits. Epoxy finds its own level across a flat, level surface, not across a substrate that slopes. Shim the workpiece level before you mix anything.
- Buying by weight. Epoxy is quoted, mixed and applied by volume. Density varies between formulations, so a weight conversion adds an error for nothing.
- Reading the lift count as covering a deep void. The lift figure on this page divides your surface film thickness by the maximum pour depth, because that is the only depth the calculator knows. A river channel or a cavity has its own depth, which you must compare with the same data-sheet limit yourself — a 1/8 in flood coat over a 1.5 in channel is one lift only if the resin is rated past 1.5 in.
Mix ratio is measured, not estimated
The volume this calculator gives you is mixed epoxy. How you split it between resin and hardener is fixed by the product — commonly 1:1, 2:1 or 3:1 by volume, sometimes a quite different ratio by weight — and an epoxy that is off-ratio does not cure soft in proportion, it can fail to cure at all. Ratios by weight and by volume are not interchangeable, because the two components have different densities. If your product states a weight ratio, weigh it; if it states a volume ratio, use graduated cups. The resin mould volume and mixing calculator splits a batch either way.
Related calculations for a resin project
A river table needs three numbers before you buy anything: the epoxy volume from this page, the timber, and the mould. For the timber, board feet converts a rough slab into the unit lumber is priced in, and wood moisture content tells you whether the slab is dry enough to pour against at all — epoxy poured against wet timber traps moisture permanently and the joint will show it. Wood shrinkage and movement matters afterwards, because timber moves seasonally and cured epoxy does not.
If you are casting into a silicone mould rather than coating a surface, the volume question inverts: you need the mould material as well as the resin, and the silicone mould material calculator handles the displacement arithmetic. For small castings where you mix in grams rather than gallons, the resin mould volume calculator is the better tool.
Finally, treat coverage and cost as separate decisions. General-purpose casting epoxy and a UV-stable table-top system are priced differently enough that the cheaper one is a real temptation, and it is the wrong saving on anything that will see daylight — an ambered top cannot be corrected without stripping it back. Enter your own delivered price per mixed gallon rather than a list price. The material cost here tells you what a pour costs today; whether that pour lasts is a question about the resin's formulation and about how much UV the finished piece will see.