Garage Floor Epoxy Calculator

Bare concrete is porous, and grinding opens it further — a floor profiled to ICRI CSP 2–3 has noticeably more real surface area than its footprint. The first coat wicks into that texture instead of sitting on top of it, so 10–20% extra is normal and a soft, very absorbent, or freshly ground slab can take 25%. That is exactly what the porosity waste field covers. Many systems handle this by specifying a thinned or dedicated penetrating primer first, which seals the surface so the build coat spreads at close to its theoretical rate.

Yes — vapor drive from below is the number one cause of epoxy peeling off, and no amount of prep fixes it. The two standard tests are ASTM F1869 (anhydrous calcium chloride), where most coatings want no more than about 3 lb per 1,000 ft² per 24 hours, and ASTM F2170 in-situ relative humidity probes, commonly capped near 75–80% RH. A quick ASTM D4263 plastic-sheet test — tape an 18 × 18 inch square down overnight and look for condensation — is a useful screen. Slabs without a vapor barrier or below grade often need a moisture-mitigating primer. Follow the product data sheet.

Diamond grinding is the professional standard. It removes laitance, curing compounds, old sealers, and paint, and leaves a consistent CSP 2–3 profile the epoxy can key into. Acid etching only works on bare, sealer-free concrete: it does nothing to a sealed or oil-contaminated floor, needs thorough rinsing and neutralizing, and residual acid or dust is a common cause of bubbles and adhesion failure. A quick check — sprinkle water on the slab. If it beads instead of soaking in, the floor is sealed and etching will not touch it. Rent a grinder with a dust vacuum.

A 100% solids coating cures at essentially the thickness you apply, so 10 mils wet dries to about 10 mils — that is the assumption behind this calculator. Water-based and lower-solids products lose their carrier as they cure: a 50% solids coating applied at 10 mils wet ends up near 5 mils dry, so you need roughly twice the gallons for the same finished build. The trade-off is working time. 100% solids often has a pot life of only 20–40 minutes at 70°F, and it drops sharply as temperature rises, so mix only what you can spread.

For a 20 × 20 ft floor (400 ft²) at 10 mils per coat, coverage is 1,604 ÷ 10 = about 160 ft² per gallon, so two coats work out to 5 gallons, and 15% for porosity brings it to about 6 gallons. Big-box kits advertising "covers a two-car garage, 500 ft²" are a much thinner roll-on film — often 2–3 mils dry — which is why one kit does not match this figure. Decide the film build you actually want first. Treat this as a planning estimate and follow the manufacturer data sheet for mix ratio and recoat windows.