Epoxy calculator for cracks, knot holes and voids

Filling cracks, knots and bark pockets in a slab uses far less epoxy than a pour — often a few ounces — so the smallest batch you can mix matters as much as the volume.

How the void volume is worked out

Cracks, knot holes and bark pockets aren’t boxes — they taper. Measuring the widest and deepest point and multiplying out would overstate the resin, so each kind of void gets a shape factor: the share of that box it actually fills.

crack  = count × length × width at top × depth × 0.5
knot   = count × diameter × diameter × 0.785 × depth
void   = count × length × width × depth × 0.6
mix    = voids × (1 + soak-in %), at least the minimum batch

• Cracks (0.5): a drying check is widest at the surface and closes to nothing at the bottom — a wedge, half the box.
• Knot holes (0.785): a round hole is π ÷ 4 of the square drawn around it. Loose knots that have fallen out often leave a hole this shape.
• Irregular voids (0.6): bark inclusions, rot pockets and worm tracks are ragged; about 0.6 of the box is a middle-of-the-road figure. If yours is nearly square-sided, allow more.

Example with the starting numbers: two cracks 10 in long, 1/4 in wide and 1/4 in deep are 2 × 10 × 0.25 × 0.25 × 0.5 = 0.625 cu in; three knot holes 1 in across and 1/2 in deep are 3 × 1 × 1 × 0.785 × 0.5 = 1.18 cu in. That’s 1.8 cu in of voids, about 1 fl oz — 1.2 fl oz with 20% for soak-in. Because the cracks (1/4 in deep) and the knots (1/2 in deep) suit different resins, that’s two mixes, and each is rounded up to the 2 fl oz minimum batch: 4 fl oz mixed for 1.2 fl oz used.

Soak-in, overfill and the minimum batch

Bare wood wicks resin into the grain around a void, and the fill settles as trapped air escapes, so plan to top up and to leave the surface a little proud for sanding flush. The 20% default covers a sound hardwood slab; punky, spalted or end-grain wood can drink far more, so raise it or seal the void walls with a thin first coat.

The minimum batch is often what sets the amount on small repairs. Measuring 1:1 or 2:1 by the teaspoon is where ratio errors creep in, and some resins need a bit of mass to cure properly. If the voids take less than your minimum, the calculator mixes the minimum and tells you how much will be left over — pour it into a silicone mold or onto scrap rather than into the bin.

Which resin for which void

Depth picks the resin more than volume does. Hairline and shallow cracks up to about 1/4 in (6 mm) deep want a thin, runny epoxy that wicks down into them — a penetrating or thin-viscosity epoxy, or a tabletop epoxy warmed a little. From about 1/4 in to 1 in (6–25 mm), a casting resin fills knot holes without overheating. Deeper voids need a deep-pour epoxy, which may also have a minimum depth. These are rules of thumb: check the maximum and minimum pour depth on your label.

Before you pour

• Blow or vacuum dust out of every crack — dust stops the resin bonding to the walls.
• Tape the back of any hole that goes through the board, and the ends of cracks that run out of an edge.
• Warm the resin bottles in warm water for thinner, bubble-free flow, and pass a heat gun or torch over the fill to pop bubbles.
• Fill the voids before a flood coat: the tabletop epoxy calculator handles the coat over the top, and the river table calculator handles a full river channel.