The mistake that wastes a kit: filling the resin for the entire form. Your slabs take up most of that space. This subtracts the wood so you buy for the channel that's actually empty.
Total width is edge-to-edge across both slabs and the river. Pour depth is how high the resin fills — usually the slab thickness.
Live edges wander. Measure each slab's solid width every 6 in and average it. This is easier and more accurate than measuring the wavy gap.
Measure the open gap between the slabs at evenly spaced points along the length. Leave boxes blank if you took fewer readings — the calculator averages the ones you fill in.
The resin only fills the open channel, so the calculator finds that empty space, not the whole slab. In slab mode it takes the form's inside width and subtracts both slabs' average widths to get the average river width, then multiplies by length and pour depth. In gap mode it averages the widths you measured at each station and does the same. One US gallon of mixed epoxy fills 231 cubic inches; one fluid ounce fills 1.805 cubic inches.
River tables are poured thick, so use a deep-pour epoxy rated for the depth you need. Standard table-top epoxy poured this deep traps its own curing heat and can crack, yellow, or flash-cure. Deep-pour formulas are designed to release that heat slowly.
Brushing a thin coat of epoxy onto the live edges before the main pour seals the pores, stops air bubbling up out of the grain, and reduces how much resin soaks in. If you skip this step, budget nearer the 15% soak-in allowance and expect more bubbles.
No. Measure only the void you want filled to the finished height. If you deliberately overfill to sand back, add a little pour depth rather than counting the entire form. It is normal to have a small amount of resin left over — running short mid-pour is the outcome worth avoiding.