Coverage is just volume
Epoxy coverage comes from a volume identity: the coating has to fill area times dry-film thickness. Convert the floor to square feet, convert mils to inches (1 mil = 0.001 in), multiply by 144 to reach cubic inches, then divide by 231 (the cubic inches in one US gallon). Multiply by the number of coats, then by (1 + waste percent / 100) for porosity, roller loss and vertical surfaces.
Manufacturer technical data sheets already state theoretical coverage at a named dry-film thickness. This calculator rebuilds that number from first principles so you can change mils, coats and waste without hunting for a different row on the sheet. Always compare the result to the product TDS before you buy; some systems quote wet-film thickness or a spread rate in square feet per gallon, not mils.
Floor area starts as length times width. If you already know the room in square feet, enter length and width that multiply to that area (for example 10 × 14.4 for 144 sq ft), or open the square footage calculator when the plan is an L-shape or has multiple rectangles.
Mix ratio and a garage-floor example
Most flooring and coating epoxies are sold as a resin and a hardener that must be mixed by volume or by weight. This tool splits the mixed total by volume parts (2:1, 1:1, 3:1 or 4:1). If your TDS specifies weight ratio, convert with the product densities or follow the kit's marked cups in place of this split.
Worked example: a 12 ft by 12 ft garage bay, two coats at 10 mils dry film each, 10% waste, 2:1 mix. Area = 144 sq ft. Thickness per coat = 0.010 in. Volume per coat = 144 × 144 × 0.010 = 207.36 cu in = 0.898 gal. Two coats = 1.795 gal. With 10% waste: 1.975 gal mixed. Resin = 2/3 ≈ 1.32 gal; hardener = 1/3 ≈ 0.66 gal.
Concrete and asphalt volume jobs use cubic yards, not mils. Concrete and asphalt estimate pour or paving tonnage; this page stays on thin-film coatings where thickness is measured in thousandths of an inch.
Waste, kits and what to verify on the can
A 5–15% waste factor is common for smooth, sealed slabs; porous or cold concrete, broadcast flake systems and vertical cove base run higher. Kit sizes are often 1.5 gal or 3 gal combined; round up to whole kits after you have the mixed total, and keep resin and hardener in the same kit so the ratio stays correct.
Check the TDS for recoat windows, maximum film build per coat, primer requirements and whether the stated coverage is theoretical or practical. Temperature and humidity change pot life and can change how far a gallon actually goes. This calculator does not model temperature.
Related planning tools: topsoil when you are preparing a bed before a coating job outdoors, and concrete volume when the substrate itself is a new pour, not a thin film.