Floor area, not wall area
This calculator multiplies length × width — the floor footprint — by the number of coats. Typical water-based and 100%-solids epoxy kits cover 300–400 sq ft per gallon per coat, but the number on your kit's label wins over any rule of thumb: solids content varies a lot between a big-box one-part kit and a professional 100%-solids system.
Preparation eats half the project
Epoxy bonds to clean, profiled concrete and to nothing else. Oil spots need degreaser, mill glaze and old sealer need grinding, and every crack needs filling before the clock starts. The kit's pot life is short — often 30–60 minutes once mixed — so the floor must be ready before you open Part B.
Two thin coats beat one thick
Unless the label says otherwise, plan a base coat plus a topcoat, which is why the calculator defaults to a coat field you can change. Garage floors that see hot tires also want the anti-slip additive broadcast into the topcoat.