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Epoxy Floor Calculator Guide
Epoxy coverage is fixed by physics, not by the marketing on the bucket. This works it out from the one constant that matters.
A US gallon is 231 cubic inches. Spread one mil thick (1/1000 inch), it covers 231 ÷ 0.001 ÷ 144 = 1,604 sq ft.
Gallons = area × mils ÷ 1,604
A 400 sq ft garage at 10 mils needs about 2.5 gallons per coat.
Mils, Not Square Feet
Every "covers 500 sq ft!" claim on a bucket is a claim about thickness in disguise. The same gallon covers 1,604 sq ft at 1 mil, 160 sq ft at 10 mils, or 16 sq ft at 100 mils.
So a product boasting big coverage is telling you it goes on thin. That's not automatically bad — but it means the coverage number and the durability number are the same number, and you can't have both.
Solids Percentage Is the Catch
100% solids epoxy cures at the thickness you apply it. Solvent-based and water-based epoxies are partly carrier that evaporates — so a 50% solids product applied at 10 mils wet cures at about 5 mils dry.
That's the trick behind cheap kits with impressive coverage figures. Half of what you rolled on left as vapour. Enter the solids percentage from the data sheet and this accounts for it; if you can't find the figure, that's informative in itself.
Bare Concrete Drinks the First Coat
Porous, unsealed concrete absorbs a meaningful share of the first coat before any film forms. This is why the first coat always seems short and why a primer coat exists.
Add 15–20% on a bare porous slab. On a previously sealed or already coated floor, 10% is usually plenty. Running out mid-floor is genuinely bad — epoxy has a pot life and you cannot pause, drive to the store and resume without a visible line.
Preparation Is the Whole Job
Epoxy does not stick to a smooth, sealed or dirty slab. It needs a mechanical profile — grinding or acid etching — and it needs the slab to be dry.
Nearly every failed garage floor you've seen peeling in sheets failed here, not in the mixing. Do a moisture test: tape a square of plastic sheet down for 24 hours and look for condensation. Coat a slab with moisture moving through it and the epoxy will lift, however well you mixed it.
Pot Life Is Not a Suggestion
Once mixed, you have a working window measured in tens of minutes, and it shortens as temperature rises. Mixing the whole kit on a warm day and then working carefully is how people end up with a bucket of solid epoxy.
Related: cubic yard calculator, deck stain calculator, square feet to cubic yards calculator.
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