Updated July 18, 2026
The math, worked first
Start with the room in the hero line above: 12 by 10 feet, 8 foot ceilings, one door, two windows. Perimeter comes to 2 times (12 plus 10), 44 feet, times 8 feet of height for 352 square feet of raw wall. Pull out the door (about 21 square feet) and the two windows (about 15 each), and you are down to 301 square feet you'll actually roll paint onto. Two coats doubles that to 602 square feet of coverage. Divide by 350 and the answer lands at 1.7, so the calculator rounds up to 2 gallons, leaving a little in the can for touch-ups later.
The arithmetic behind that: wall area equals perimeter (2 times length plus width) times ceiling height, unless you type a wall area directly into the calculator. Subtract roughly 21 square feet per door and 15 per window to reach the paintable area, multiply by the number of coats, then divide by 350 square feet per gallon and round up.
Primer or topcoat: they don't cover the same
People plug the topcoat number in for every gallon and end up short. Primer runs thinner and soaks into bare drywall, fresh patch compound, and old flat paint, so it covers less per can, somewhere around 200 to 300 square feet rather than 350 to 400. Skip priming on a strong color change (say, a deep red going to white) and you'll be back at the store for a third coat that a primed wall would not have needed. A tinted primer matched toward your topcoat color often buys you back that third coat. Bare drywall, patched drywall, and anything more than a shade or two of color change all call for primer first; repainting the same color over a sound existing coat usually does not.
Coverage this calculator uses
| Constant | Value used here |
|---|---|
| Topcoat coverage per gallon, per coat | 350 ft² |
| Door opening subtracted | 21 ft² each |
| Window opening subtracted | 15 ft² each |
| Primer coverage (for reference, not in the calculator) | 200 to 300 ft² per gallon |
The 350 ft² figure and the primer range come from Sherwin-Williams' paint calculator, which most manufacturer labels track closely.
How to measure a room for paint
Multiply the room perimeter by the ceiling height for total wall area, or measure each wall and add them up. Subtract the big openings, roughly 21 square feet per door and 15 per window, though your own doors and windows might run a bit different. Skip subtracting trim, outlets, or small fixtures; they're too small to move the number and the rounding absorbs them anyway. Painting a ceiling or an odd-shaped wall? Get the raw area with the square footage calculator and drop it straight into the wall area field here.
Tips from the field
- Buy from the same batch and, for large jobs, box your paint by pouring all the cans into one bucket and mixing. It evens out any slight tint variation between cans.
- Round gallons up. The leftover handles touch-ups and you avoid a second store run for half a wall.
- Primer is not optional on bare drywall, fresh patch, or a big color change. It seals the surface and cuts the number of finish coats.
- Keep the lid label or the can; the formula on it lets the store rematch your color later.
Paint material list
Room: . Coats: . Doors: . Windows: .
Paintable area: . Total coverage needed: . Gallons to buy: .
Other calculators for this job
Shop paint & supplies
Interior paint, primer, rollers and tape delivered to your door.