Epoxy Grout Calculator — Kit Volume Math & Premixed Buckets

Epoxy is sold as liquid volume, so the calculation is volume against volume. First compute the joint volume: depth × (2 × tile side × joint width + joint width²) ÷ (tile side + joint width)², all in millimetres, gives litres per square metre — divide by 10.76 for litres per square foot. Multiply by your area, add about 10% for waste and clean-up, then divide by the kit's mixed yield. A SPECTRALOCK PRO Full unit yields 3.0 L, a Mini 0.7 L, a Commercial box 12.0 L, and a 9 lb Kerapoxy CQ kit about 2.4 L. One example: 100 ft² of 12 × 12 × 3/8 in tile at 1/8 in joints holds 1.87 L; with 10% waste that is 2.06 L — one Full kit (3.0 L) or one Kerapoxy kit (2.4 L) covers it.

The formula behind every epoxy tile grout calculator: joint volume (L) = area (m²) × depth (mm) × (2 × side (mm) × joint (mm) + joint²) ÷ (side + joint)². For a square tile this is exactly the open channel area between four tiles, times the joint depth. Kits then divide out: kits = joint volume × 1.1 ÷ kit yield. Pre-mixed pails use the same formula with the pail's litres instead of a kit's — 1 US gallon is 3.785 L. What you never do with epoxy is compute bag weight from a cement density: there is no dry powder to weigh, and the mixed material has no water to lose.

Divide your joint volume by the pail's liquid volume. A 1 gallon pail holds 3.785 L, and the printed coverage charts agree: Custom Building Products' SimpleGrout table lists 199 ft² per gallon at 12 × 12 × 3/8 in tile with 1/8 in joints — the pure volume math gives 204 ft², within 3%. Mapei's Flexcolor CQ lists 155 ft² per gallon at 6 × 6 × 1/4 in with 1/8 in joints, which the same arithmetic reproduces at 151 ft². So for pre-mixed, the answer is simply: area × litres-per-ft² ÷ 3.785, rounded up to whole pails.

Work space by space, because joint geometry changes per surface. Measure each space's area, pick the tile size, joint width and tile thickness for that space, compute its litres of joint (the formula above), sum the spaces, then divide by 3.785 L per gallon and round up. A typical bathroom: 60 ft² floor of 12 × 12 × 3/8 at 1/8 in = 1.11 L; 90 ft² walls of 8 × 8 × 3/8 at 1/8 in = 2.43 L; 30 ft² shower floor mosaic of 2 × 2 × 1/4 at 1/8 in = 2.02 L. Total ≈ 5.6 L → two 1 gallon pails with margin — which is exactly why resealable pre-mixed packaging is forgiving for multi-space jobs.

Pot life decides batching, not quantity — but it changes how you buy. SPECTRALOCK PRO gives 80 minutes of working time at 21 °C per mix; Kerapoxy CQ about 45 minutes at 73 °F. You cannot mix three Full units in one bucket and place them all: the third would be setting while the first is still on the wall. So the planner counts mixing windows — one kit per window for Minis and Fulls, and the Commercial 12 L box is deliberately four separate Full doses for exactly this reason. If your schedule only allows two windows, buy the packaging that fits two windows, not more material than you can place.

The joint volume is identical — geometry does not care about chemistry. What changes is the conversion. Cement grout is sold as dry powder: litres of joint × ~1.6 kg/L of mixed grout, adjusted for the water you add. Epoxy is sold as liquid volume: litres of joint ÷ litres per kit or pail, full stop. Epoxy also loses nothing to evaporation, so the volume you compute is the volume you place, while a cement joint slurry shrinks slightly as it cures. That is why epoxy coverage charts read a touch more generous than cement tables for the same tile.

A 2-part kit pairs liquid resin and hardener with a coloured powder (Part C) that fills volume and carries the pigment — SPECTRALOCK PRO's Full unit is A 18.1 fl oz + B 18.6 fl oz + 9 lb of powder, mixing to 3.0 L. A 3-part kit like Davco's or Kerapoxy CQ splits resin, hardener and filler into three containers but works the same way. The part count is packaging, not math: both resolve to one number, the mixed litres printed as yield. For coverage purposes a 9 lb kit and a 2.4 L yield describe the same bucket of mixed epoxy.

Not always — and the distinction matters for where you can use it. True epoxy grouts are reactive 2-part systems that cure chemically and are rated for submerged use in pools. Some pre-mixed pails, like SPECTRALOCK 1 Translucent, are engineered to meet ANSI A118.3 — the epoxy performance standard — in a resealable single-component pail, and LATICRETE lists it as submersible after 14 days. But most pre-mixed grouts (SimpleGrout, Flexcolor CQ, Easy Grout-style products) are acrylic-based: excellent for showers, splashbacks and interior floors, but explicitly not for submerged areas, steam rooms or continuous heat. Check the rating on the pail, not the word 'pre-mixed' on the shelf.