How Much Epoxy to Fill Wood Cracks, Knots and Voids?

Updated 2026-10-19

Cracks, checks, knots and bark voids can become design features once filled with clear or tinted epoxy — but only if you mix enough resin to fill them without a frantic second batch. Unlike a flat flood coat, crack volume is irregular. This guide shows practical ways to estimate that volume, convert it to gallons, and choose a product thick enough for the deepest section.

For layered deep fills, use the deep pour epoxy calculator. For simple rectangular approximations, the main epoxy calculator works too.

Think in volume, not "a little bit"

Epoxy fills space. The amount you need is the volume of the empty cavity (plus what soaks into porous end grain and plus waste in the cup). Even a crack that looks narrow can add up when it runs the length of a slab and goes deep.

Volume (in³) ≈ average length × average width × average depth
Gallons = in³ ÷ 231
Fl oz = in³ ÷ 1.8047
Liters = in³ × 0.016387

Method 1: Measure as a rectangular slot

For a long check or separation:

Measure the crack length along the surface (inches).
Measure width at several points; average them.
Probe depth at several points with a thin ruler or wire; use a realistic average, or use the maximum if you will fill to that depth everywhere.
Multiply length × width × depth for cubic inches.

Example: A check averages 40 in long, 0.25 in wide and 1.0 in deep → 40 × 0.25 × 1.0 = 10 in³ ≈ 0.043 gal ≈ 5.5 fl oz. With 20% soak-in/waste → about 12 in³ ≈ 6.6 fl oz.

Method 2: Knots and odd voids

Treat a knot hole roughly as a cylinder or cone:

Cylinder: π × (diameter ÷ 2)² × depth
Cone / tapered void: π × (diameter ÷ 2)² × depth ÷ 3

Example: A knot opening 2 in across and 1.5 in deep, treated as a cylinder: π × 1² × 1.5 ≈ 4.71 in³ ≈ 2.6 fl oz. Add buffer for surrounding soft fiber that will drink epoxy.

Method 3: Water or sand displacement (when practical)

If the void can be temporarily sealed or the piece can be oriented so liquid stays put, you can fill the cavity with water and weigh or measure it — similar to the mold water method. Dry the wood completely afterward; moisture and epoxy are a bad pair. Dry sand or rice can work for a rough volume estimate when you must avoid wetting the wood, but packing voids makes them less precise than water.

Method 4: Multiple cracks on one slab

Estimate each major void, add them, then add a shared waste percentage. Tiny hairline surface checks that you only spot-fill with a brush may be negligible compared with a through-check — focus measuring time on the large cavities.

Deep pour vs tabletop for cracks

Depth drives product choice:

Shallow surface cracks (a few millimeters) often use a sealing or tabletop epoxy in thin applications.
Deep checks and voids usually need a deep pour / casting epoxy, poured in layers within the product's maximum depth per pour.

Pouring coating epoxy into a deep cavity can trap heat and cause cracking, yellowing or bubbles. Read deep pour vs tabletop epoxy and your TDS maximum pour depth before you mix.

Worked example: slab with crack + knot

Crack: 36 × 0.3 × 1.25 in = 13.5 in³
Knot (cylinder): π × (1.25)² × 1.0 ≈ 4.9 in³
Subtotal: 18.4 in³
20% buffer: 22.1 in³
Gallons: 22.1 ÷ 231 ≈ 0.096 gal
Fluid ounces: 22.1 ÷ 1.8047 ≈ 12.2 fl oz
Liters: 22.1 × 0.016387 ≈ 0.36 L

If the deepest section is 1.25 in and your deep pour epoxy allows 1 in per layer, plan two layers and split the volume accordingly with the deep pour calculator.

Seal first when wood is porous

End grain, punky knot fibers and bark inclusions can keep releasing air into fresh epoxy. Many woodworkers brush a thin seal coat on crack walls, let it gel or cure per the TDS, then fill. That seal coat uses a small extra amount of epoxy — budget it. Details on thin vs thick coats: seal coat vs flood coat.

Color, glitter and additives

Colorants and metallic powders displace a little volume and can affect heat and cure if overdosed. Follow the manufacturer's limits for additives. When in doubt, mix a small test batch in a similar thickness.

Containing the pour

Tape the bottom of through-cracks, use temporary dams, or fill on a sealed form so epoxy does not drain onto the bench. Leaks are a common reason people under-order resin — the volume left the crack.

Common mistakes

Underestimating depth. Surface width looks small; the cavity below may be larger.
No buffer for soak-in. Dry, cracked wood can absorb more than a sealed surface.
Wrong epoxy for thickness. Match product to depth.
Mixing only "what looks right" in the cup. Measure; small voids still need correct A/B ratios.
Trapping moisture. Let wood reach a moisture condition appropriate for your climate and finish schedule; follow product guidance on substrate condition.

Shopping list tips

Kit labels may show combined volume. Round up to the next size that covers your buffered total. Leftover sealed epoxy may have a limited shelf life — check storage instructions on the TDS.

Safety

Wear PPE per the SDS, ventilate, and dispose of leftover mixed epoxy as directed. Never pour waste epoxy down drains.

Tinted fills vs clear fills

Opaque or heavily tinted epoxy can hide dust and slight mismatches in a crack, while clear fills show every bubble and fiber. Either way, the volume math does not change — colorant usually adds only a small displacement. Stay within the pigment loading your TDS allows so cure and heat behavior stay predictable.

Order of operations on a slab

Stabilize and dry the slab as appropriate for your shop climate.
Clean dust and loose fiber from cracks.
Tape or dam the bottom of through-voids.
Optional: brush a seal coat on crack walls; wait for the recoat window.
Mix deep pour epoxy for the fill volume plus buffer; pour in layers if required.
After full cure, flatten, sand and decide on a flood coat or other finish.

That final flood coat is separate epoxy — calculate it with the tabletop calculator like any other topcoat.

Summary

Estimate crack and void epoxy by approximating cavities as slots, cylinders or cones, convert in³ to gallons with ÷ 231, add 15–25% when wood is porous, and choose deep pour epoxy for thick fills in layers. Seal porous walls when needed, contain leaks, and verify mix ratio and max depth on the TDS. Run the numbers through the deep pour epoxy calculator before you open the kit.