Paint is sold by the gallon and spread by the square foot, and the gap between those two numbers is where people end up making a second trip. Enter the room, get gallons — walls, ceiling and trim counted separately, because they take different amounts.
Counted at 21 sq ft per door and 15 sq ft per window — standard sizes. Skip this if you are painting the doors too.
Why the number on the can is optimistic
Every manufacturer prints something like 350 to 400 square feet per gallon. That figure assumes a smooth, previously sealed, similar-coloured surface, a full-nap roller, and someone who is not stretching the paint to make it last. Change any one of those and the real number drops.
The three things that actually eat paint are porosity, texture and colour change. Bare drywall and fresh joint compound are porous, so the first coat soaks in rather than sitting on top — that is what primer is for, and skipping it costs more in finish paint than the primer would have. Texture adds surface area your tape measure cannot see. And going light over dark is a hiding problem: no number of thin coats fixes it, but a tinted primer usually does in one.
Buy it all at once
Tinted paint is mixed to a formula, but the base can vary batch to batch. If you buy two gallons now and a third next week, there is a real chance you will see the join on a long wall in daylight. Buy everything for a room in one go and ask them to box it — pour the cans together into a larger bucket and stir — so the whole room comes off one consistent mix.
Sheen changes the maths too
Flatter paints hide surface flaws and touch up well; glossier paints wipe clean but telegraph every roller mark and every patch you feathered badly. That decision is worth making before you buy, not after — we went through it the hard way in the sheen guide and why ceilings get flat paint.
More on the parts this calculator cannot do for you: what interior painting costs, cutting in without tape, back-rolling, and latex versus oil.
Method
Wall area is perimeter × height, less 21 sq ft per door and 15 sq ft per window. Spread rates are 375 sq ft/gal on smooth previously painted surfaces, 300 on bare or patched drywall, 260 on textured walls — the low end of published manufacturer ranges, because published ranges assume ideal conditions. Light-over-dark adds one coat's worth of paint and triggers a primer line. Trim is estimated from perimeter plus 20 sq ft per door and 12 sq ft per window at 350 sq ft/gal. These are planning figures, not a guarantee; buy the calculated amount and keep the receipt for the last can.