How Much Epoxy for a River Table?

Any pour with vertical sides is a box: volume equals length times width times depth. Work in one unit throughout and convert at the end. In imperial units the result is in cubic inches, and dividing by 231 gives US gallons, because the US gallon is defined as exactly 231 cubic inches. In metric units, cubic millimetres divided by 1,000,000 give litres. One US gallon is 3.785411784 litres and holds 128 US fluid ounces.

Depth means the full depth of epoxy from the floor of the mould to the finished surface, including any amount left proud for flattening. If the slabs are thinner than the pour, or the epoxy is to cover them with a flood coat, that extra layer over the wood is a separate box to add.

Live edges wander, so the river is rarely the same width along its length. A practical approximation is to measure the gap between the slab edges at several evenly spaced points, average those widths, and treat the river as a rectangle of that average width. More measurement points give a better average, especially where the river necks down or opens into a wide pool.

Measure at the widest point through the thickness as well. Live edges often undercut, so the river can be wider at the bottom of the slab than at the top. The averaging method assumes vertical sides; where the edges slope or undercut significantly, measure at mid-depth or add to the waste allowance.

A 60 in long river is measured at four stations: 5, 7.5, 10 and 6.5 in. The average width is 29 / 4 = 7.25 in. At 1.5 in deep, the volume is 60 × 7.25 × 1.5 = 652.5 cubic inches, which is 652.5 / 231 = 2.825 US gallons or 10.69 litres.

With a 10% allowance the order becomes 3.11 gallons (398 fl oz). One manufacturer's river-table kit instructions call for a 1:1 mix by volume and no more than 1/2 in per layer. Using those published limits, the pour needs 3 layers of 0.50 in, about 1.04 gallons of mixed epoxy each, and the total splits into 1.55 gallons of resin and the same of hardener. A different product would give a different layer count.

Measuring the inside of the mould is often easier than measuring the river, but the mould volume includes the slabs. The wood occupies that space, so the epoxy needed is the mould volume minus the volume of wood below the finished surface. On a typical river table the slabs are most of the mould, which makes this subtraction the largest single correction in the whole estimate.

Consider an 1800 × 900 mm mould poured 45 mm deep. The box holds 72.9 litres. Two slabs, each 1800 × 350 mm on average and fully submerged, displace 56.7 litres, leaving 16.2 litres of river. Ordering for the empty mould with the same 10% allowance would mean 80.2 litres instead of 17.82 — 4.5 times too much.

When slab shapes are too irregular to measure as rectangles, estimate the share of the mould area covered by wood from a top-down photo or a sketch on grid paper, and subtract that percentage instead. Only the thickness of wood below the epoxy surface displaces resin; a slab that stands proud of the pour displaces only up to the pour depth.

Wood is porous and contains air. When liquid epoxy covers it, that air can escape into the pour and rise as bubbles that are then trapped as the resin thickens. Manufacturers' river-table instructions therefore call for thin seal coats of mixed epoxy on the wood before the main pour; one set of kit instructions specifies two thin coats to stop air in the wood off-gassing into the pour. Temperature matters too: epoxy guidance from one manufacturer notes that applying cold epoxy to warm wood can cause air in the fibres to expand, producing bubbles, craters and pitting.

Seal coats are thin, but they cover every face of the slab that the pour will touch, including end grain, which absorbs more. Their volume is small compared with the river, so it is reasonable to include it in the waste allowance rather than calculate it separately, unless the slabs are very large or very porous.

Epoxy cures by a chemical reaction that releases heat. WEST SYSTEM explains that the larger or thicker the mass of mixed epoxy, the more heat it generates and the more likely an uncontrolled cure becomes, and advises filling large cavities in several layers rather than one thick layer. Deep-pour casting resins are formulated to release heat slowly so that thicker layers are possible, while coating epoxies are intended for thin films.

This is why the maximum depth per pour is a property of the product, not of the project, and why it must be read from the data sheet of the resin actually being used. The calculator asks for that figure and does not offer a default. Data sheets may also specify a minimum time between layers and a window for recoating without sanding, and these govern how long a multi-layer pour takes.

Two-part epoxies are mixed at a fixed ratio of resin to hardener, such as 1:1 or 2:1. The ratio can be stated by volume, by weight, or both, and the two are generally not the same number because resin and hardener have different densities. Measure in the same terms as the published ratio: cups and graduated containers for a volume ratio, a scale for a weight ratio.

To split a total volume, divide it in proportion to the parts. For a 2:1 ratio by volume, resin is two thirds of the total and hardener one third. The calculator applies the ratio to the total including the waste allowance, so both containers are sized for the full order.

Some mixed epoxy is always lost: the film left in mixing containers, small leaks at the mould seams, resin absorbed by the wood beyond the seal coats, and a slight overfill so the surface can be flattened. The right allowance depends on the mould, the number of batches and the product. More batches mean more mixing-cup residue.

If the finished weight matters, for moving the table or sizing a base, multiply the volume by the mixed density on the data sheet. Densities are published per product, so the calculator uses only the value entered and does not assume one.

Each step below changes the volume or the number of pours.

The Epoxy Resin Calculator performs these steps for rectangular moulds or measured rivers, subtracts slabs by dimensions or percentage, and reports litres, US gallons and fluid ounces, resin and hardener volumes, the number of layers and an optional weight from the density you enter.