A gallon of paint covers about 350 square feet per coat. To find your gallons: add up the wall area (perimeter × ceiling height), subtract about 21 square feet per door and 15 per window, divide by 350, and multiply by your number of coats — then round up to whole gallons. A typical 12×10 bedroom with two coats needs 2 gallons.
Why paint estimates go wrong in both directions
Underestimate, and you're making a second trip to the store mid-project with paint on your clothes, hoping the new gallon's tint matches the first — it usually does, but "usually" is doing heavy lifting at hour four of a Saturday. Overestimate, and you're storing a $45 gallon in the garage for a decade "for touch-ups" you'll do with a brush-load, not a gallon.
Both errors come from the same shortcut: measuring the room as if it were four solid walls. Real rooms have doors and windows that don't get painted, and in a small room those openings are a meaningful share of the area — in the worked example below they're 51 of 352 square feet, nearly 15%. Skip the deductions and you over-buy; skip the second coat in your math and you under-buy. This estimator does the full sequence a painter does: gross area, minus standard deductions, divided by realistic coverage, times coats, rounded up at the very end — never before, because rounding early compounds.
How the estimate is calculated
- Wall area: perimeter × wall height — 2 × (length + width) × height. If you're painting the ceiling, add length × width.
- Deductions: 21 sq ft per standard door (roughly 3 ft × 7 ft) and 15 sq ft per average window — the same allowances painters and paint-can charts use.
- Gallons per coat: paintable area ÷ coverage per gallon. The default is 350 sq ft, the figure printed on most cans for smooth, primed walls. Drop it to 250–300 for rough texture, brick, or unprimed surfaces.
- Total and purchase: gallons per coat × number of coats gives the exact amount. The "gallons to buy" rounds that up to whole gallons, because paint is sold by the gallon — while the exact figure stays visible so you can judge whether a quart top-up makes more sense.
Worked example
A bedroom is 12 ft × 10 ft with 8 ft walls, 1 door, 2 windows, getting 2 coats, ceiling not painted, standard 350 sq ft coverage:
- Wall area: 2 × (12 + 10) × 8 = 352 sq ft
- Deductions: 1 door × 21 + 2 windows × 15 = 51 sq ft
- Paintable area: 352 − 51 = 301 sq ft
- Per coat: 301 ÷ 350 = 0.86 gal
- Two coats: 1.72 gal exactly → buy 2 gallons
Add the ceiling and the same room becomes 421 paintable sq ft (the ceiling adds 120, minus nothing — ceilings have no doors), which is 1.20 gal per coat, 2.41 gal total → buy 3 gallons. That single toggle is the difference between a two-gallon and a three-gallon project, which is why "are you doing the ceiling?" is the first question a painter asks.
Primer: the coat nobody budgets for
Paint is formulated to cover paint. On new drywall, bare wood, water stains, or patched areas, the first coat soaks in unevenly and the finish looks blotchy no matter how many coats follow. That's the primer's job: it seals the surface so the paint can do its job evenly. Primer is a separate, usually cheaper product, and it covers roughly the same 350 sq ft per gallon — budget about one gallon of primer for every gallon of first-coat paint when the surface needs it. For a dramatic color change (dark to light especially), a primer tinted toward your new color can save an entire coat of the expensive stuff. The estimator's paint figures are for paint only; if your walls are bare or your color change is dramatic, add the primer on top.