River table epoxy calculator

How the calculator works

A river table void is close to a box: the river runs the length of the slab, it has some width, and you fill it to a set thickness. The volume of a box is length × width × height, so the only hard part is choosing a width that represents a river whose edges wander.

The calculator gives you two ways to do that. You can type one average width, or you can measure the river at even intervals along its length and enter every measurement. With a list, the calculator averages the numbers for you. Five or six measurements on a curving river give a much better average than a guess from the widest and narrowest points.

The result is then converted to the units you buy resin in. One US gallon is defined as exactly 231 cubic inches, and one inch as exactly 2.54 cm (NIST Handbook 44, Appendix C), so no rounding enters the conversion.

Formula and assumptions

V = L × W_avg × T × (1 + void %) × (1 + loss %)

• L is the river length inside the form.
• W_avg is the average river width, typed directly or averaged from your measurements.
• T is the total resin thickness, measured from the form base to the finished resin surface.
• Void allowance covers resin that disappears into cracks, bark pockets and undercut live edges. It is your estimate. Clean, sealed edges need close to 0 %.
• Mixing loss covers resin left in buckets and on stir sticks. The 10 % default is an assumption for you to change, not a manufacturer figure.

The calculator assumes the form base is flat and level and that the slab edges are roughly vertical. It does not know how much resin the wood itself absorbs. Manufacturers deal with that by sealing the wood before the main pour: TotalBoat says the wood needs sealing before the main pour of a river table (TotalBoat), and EcoPoxy recommends a seal coat on every surface that will be submerged in its FlowCast casting resin, because air and moisture escaping from the wood can cause bubbles (EcoPoxy). The seal coat is a separate, thin application; size it with the flood coat calculator.

Worked example

A walnut river table with a river 72 in long. Measured every 12 in, the river is 6, 7.5, 9, 10, 8.5, 7 and 6 in wide. The pour is 2 in thick. The edges are sealed and clean, so the void allowance is 0 %. Mixing loss is left at 10 %.

1. Average width: (6 + 7.5 + 9 + 10 + 8.5 + 7 + 6) ÷ 7 = 54 ÷ 7 = 7.71 in.
2. Net volume: 72 × 7.71 × 2 = 1,110.9 in³.
3. In gallons: 1,110.9 ÷ 231 = 4.81 gal (18.2 L).
4. With 10 % mixing loss: 4.81 × 1.10 = 5.29 gal (20.0 L) of mixed resin.

Using the widest measurement (10 in) instead of the average would have suggested 6.86 gal with the same loss factor, about 1.6 gal more than needed.

Before you buy

• Check the layer limit. A 2 in pour may be one layer or two, depending on the product. The layer planner reads the maximum depth from each data sheet.
• Mixed volume is not the label volume of one part. Kits are usually sold by the combined volume of Part A and Part B. The batch splitter turns a mixed volume into Part A and Part B amounts.
• Measure the inside of the form, not the slab. If the form is wider than the river at the ends, that space fills too.