Epoxy coverage is one of the few coating numbers you can derive exactly, because epoxy is close to one hundred percent solids — what you spread is what stays. A gallon is 231 cubic inches, and spreading that one mil thick covers 1,604 square feet.
Where 1,604 comes from
One gallon is 231 cubic inches. A mil is one thousandth of an inch. A square foot is 144 square inches.
So a gallon spread one mil thick covers 231 divided by 144 divided by one thousandth, which is 1,604 square feet. Divide that by your target thickness and you have coverage.
At 10 mils, a gallon covers about 160 square feet. At 20 mils, 80. The calculator does this for whatever thickness you enter and shows the working, because this is a figure worth being able to check.
What thickness you actually want
A thin roll-on garage kit from a big box store is typically 4 to 6 mils. It is really a coating, it will show tyre marks, and hot tyre pickup can lift it. It is the cheapest option and it looks good for a couple of years.
Ten mils is a reasonable do-it-yourself target for a garage floor with a decent product. This is where most quality kits land.
Twenty mils and up is commercial territory, usually applied as a self-levelling system with a squeegee and back-rolled. It is a different product and a different budget.
The figure quoted on a product is normally the dry film thickness. With near-solids epoxy the wet and dry numbers are close, which is why the arithmetic works so cleanly here and not for solvent-based coatings.
Why fifteen percent waste
Bare concrete is porous, and the first coat loses material into the surface rather than onto it. A freshly ground slab can absorb noticeably more than a previously coated one.
Beyond absorption, there is material left in the roller, in the pan, and in the mixing bucket — and epoxy has a pot life, so whatever is in the bucket when it starts to set is finished, used or not.
Fifteen percent is a working default. On a rough or heavily profiled slab, raise it.
Pot life is the constraint on the job
Once the two parts are mixed, the clock runs — typically twenty to forty minutes depending on the product and the temperature. Warmer means faster, and a hot garage can cut it in half.
This changes how you buy and how you work. Mix only what you can lay down, work in sections, and have a second person if the floor is large. A batch that kicks in the bucket is a batch you throw away, which is why the waste figure is not conservative.
Flakes and topcoat
Decorative flakes are broadcast into the wet base coat and do not change the base coat quantity, but they do generally require a clear topcoat over them — which is a separate product and a separate calculation.
If you are doing a flake system, run this calculator twice: once for the base at its thickness and once for the topcoat at its own, usually thinner, spec.