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Running out of resin halfway through a river pour can ruin a slab you paid hundreds of dollars for. Buying three times what you need wastes money just as fast. Fortunately, the math is simple: you’re filling a box. This guide walks through the formula, three worked examples, the extra you should add for waste, and why pour depth limits matter as much as total volume.
Shortcut: plug your numbers into our epoxy calculator and it does the math below for you.
The core formula
A river is an irregular channel, but you can treat it as a rectangle using its average width:
Volume (cubic inches) = river length × average river width × depth
Then convert:
- Gallons = cubic inches ÷ 231 (one US gallon is exactly 231 cubic inches)
- Liters = cubic inches × 0.016387
- Fluid ounces = gallons × 128
Finding the average width
Measure the gap between the slabs at 5–10 evenly spaced points along the length, then average the readings. For example, readings of 3, 5, 7, 6 and 4 inches give an average of 5 inches. Taper-heavy, live-edge rivers are where people misjudge volume most, so take more measurements on wild edges.
What depth to use
Use the full slab thickness if the river runs all the way through the table, which is the usual case. If you plan a top coat or flood coat over the whole surface, calculate it separately (see below).
Example 1: Coffee table
- River: 40″ long × 4″ average width × 1.5″ deep
- 40 × 4 × 1.5 = 240 cu in
- 240 ÷ 231 = 1.04 gallons (≈ 3.9 L)
Example 2: Dining table
- River: 84″ long × 6″ average width × 1.75″ deep
- 84 × 6 × 1.75 = 882 cu in
- 882 ÷ 231 = 3.82 gallons (≈ 14.5 L)
Example 3: Wide “lake” style table
- River: 72″ × 14″ average × 2″ deep
- 72 × 14 × 2 = 2,016 cu in
- 2,016 ÷ 231 = 8.73 gallons (≈ 33 L)
Add a waste and absorption margin
Formula volume is the minimum. Real pours lose resin to:
- Seal coat: Before the main pour, brush a thin coat on bare wood to stop bubbles from rising out of the grain. TotalBoat’s ThickSet instructions specifically call for a thin skim coat on wood and other porous surfaces, cured before you add more epoxy.
- Leaks: Even well-sealed melamine forms can weep at the corners.
- Wood absorption and cracks: Bark pockets and checks soak up more resin than you’d expect.
- Leftover in mixing buckets.
- Overfill for flattening: Most makers pour slightly proud of the wood and then plane or sand it flush.
A common rule of thumb is to add 10–15%, and closer to 20% for very porous or heavily checked slabs. For Example 2, that means buying about 4.3–4.6 gallons.
Don’t forget the top coat
If you’ll flood-coat the whole tabletop after flattening, add:
Top-coat volume = table length × table width × coat thickness
A 1/16″ (0.0625″) coat on a 84″ × 40″ top works out to 84 × 40 × 0.0625 = 210 cu in ≈ 0.91 gallons. Many builders skip a flood coat and finish with oil or hardwax instead. That’s a design choice, not a requirement.
Depth limits: why total volume isn’t the whole story
Epoxy cures through an exothermic (heat-producing) reaction. Pour too deep or too much at once and the resin can overheat, which can cause yellowing, cracking, waves or even smoking. That’s why every resin has a maximum pour depth, and you may need several layers to reach your total volume.
Examples from manufacturer data:
| Resin | Max depth per pour (manufacturer) | Mix ratio | Notes |
|---|---|---|---|
| TotalBoat ThickSet (deep pour) | 1/2″ per layer for river/slab pours; layers ≥4 hrs apart | see label | Working time ~30 min at 70–77°F; full cure 3–5 days |
| EcoPoxy FlowCast | 0.5–1.5″ recommended casting thickness; max recommended volume 30 L | 2:1 by volume | Working temp 68–77°F |
| Stone Coat Countertop Epoxy | 1/8″ per application recommended (TDS max casting 0.25″) | 1:1 by volume | A coating epoxy — use for top coats, not the river |
So the 1.75″-deep dining table in Example 2 would take four layers with ThickSet (1/2″ each), or two layers with FlowCast (up to 1.5″ recommended). The total resin is the same either way. What changes is how many kits you buy at once, your mixing schedule and your timeline.
Practical tip: Split the total volume by layer so you know how much to mix each session. With ThickSet in Example 2, each 1/2″ layer is 84 × 6 × 0.5 = 252 cu in ≈ 1.09 gallons.
Temperature matters too
Both deep-pour products above specify narrow working temperatures: 65–80°F ambient for ThickSet, and 68–77°F (ideal 72°F) for FlowCast. A cold shop slows the cure and can leave resin soft. A hot shop speeds the exotherm. Measure your shop temperature before you choose a pour depth.
Weight check for your form and legs
Cured epoxy weighs about 9–9.5 lb per gallon. Check your product’s density on the TDS, since it varies. The Example 2 river adds roughly 35–40 lb of resin on top of the wood. Size your legs and hardware with that in mind.
Step-by-step: measuring your project
- Flatten and arrange slabs in the form exactly as they’ll sit.
- Measure river length along the centerline.
- Measure width at 5–10 points and average them.
- Measure depth from the form bottom to the slab top.
- Calculate volume in cubic inches and convert to gallons or liters.
- Add 10–20% for waste, the seal coat and overfill.
- Divide into layers using your resin’s maximum pour depth.
- Add the top coat if you’re planning one.
- Buy from the same batch if possible, so color and clarity match across layers.
Common mistakes
- Using maximum width instead of average width. This can double your estimate.
- Forgetting live-edge voids under the slab. Seal the bottom of the form and any through-cracks before pouring.
- Pouring coating epoxy deep. Countertop or table-top epoxies are designed for thin layers.
- Mixing by eye. Always measure by the manufacturer’s ratio, by volume or by weight as the TDS specifies.
FAQ
How much epoxy for a 6-foot river table? It depends on the width and depth of the river. A 72″ × 6″ × 1.5″ river is 648 cu in ≈ 2.8 gallons before waste, or about 3.1–3.4 gallons with a margin.
Is it cheaper to pour the river thinner? You can’t use less than the slab thickness for a through-river. Some builders fill the bottom with a cheaper filler or add rocks to reduce volume (see our river-table-with-rocks guide).
Gallons or liters? US resin kits are usually sold in gallons. EcoPoxy also lists liters. The calculator outputs both.
Sources
- TotalBoat, ThickSet Epoxy instruction sheet & TDS; ThickSet product page.
- EcoPoxy, FlowCast Technical Data Sheet (v2.3).
- Stone Coat Countertops, Countertop Epoxy TDS and product page.
- NIST Handbook 44 (US gallon = 231 cubic inches).