Measure only the river (the gap between the slabs), not the whole table. Measure the width at 3–5 points and enter the average.
Gallons = length × average width × (depth + pour proud) ÷ 231, plus waste. Example: 72 × 8 × 1.5625 = 900 in³ ÷ 231 = 3.9 gal, about 4.3 gal with 10% extra.
A 72 in river averaging 8 in wide on 1.5 in slabs needs about 3.9 gallons, or roughly 4.3 gallons with 10% for waste and seepage.
Use a deep-pour casting epoxy for the river itself; it cures slowly so it can go 1–2 in or more per pour without overheating. Tabletop epoxy is for the thin final flood coat.
Epoxy soaks into live edges and bark pockets and leaks through small gaps in the form. Seal the edges first and plan 10–15% extra.
Divide total depth by the max pour depth on your label and round up. 1.5 in of a 2 in-rated resin is one pour; with a 1/2 in-rated resin it is three.
Related: resin calculator · pour layer planner · mix ratio by weight · cure time by temperature · project cost
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