Coats, coverage and the pot-life problem
The area is straightforward — floor length × width, plus any curb, step or coved wall area you are also coating. Multiply that by the number of coats, divide by the coverage the kit claims per coat, and round up. The two numbers that actually decide whether you have enough are the coverage figure and the condition of the slab. Epoxy coverage is quoted for a smooth, properly prepared surface, and the preparation itself changes the answer: a floor that has been acid-etched or diamond-ground has an open profile that swallows the first coat, so the base coat can easily use 20–30% more than the label suggests while the top coat lands close to it. Bare, never-sealed concrete behaves the same way. If you plan to broadcast decorative flake, budget extra clear top coat as well, because the flake creates a textured surface that needs more material to bury and level.
The other constraint is time, not quantity. Two-part epoxy has a pot life — often well under an hour once parts A and B are mixed — and it starts thickening whether or not you are ready. That makes it a mistake to mix a large batch to cover a big floor in one pass. Work in sections you can roll out inside the pot life, keep a wet edge, and mix the next kit only when the previous one is nearly down. It also means you should have every kit on hand before you start: a floor coated with two different batches days apart will show the join. Buy one spare kit if your floor lands close to a whole-kit boundary, and check the recoat window on the label, since most epoxies must have the second coat applied inside a specific time range or the surface has to be abraded before it will bond.
Coverage varies by product, film thickness and surface porosity, and prepared or bare concrete uses noticeably more — always check the coverage and recoat window stated on the kit label.