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Home & Garden · 5 min read · Last updated September 2026
Worked example: 12×15 ft room, 9 ft ceilings. Perimeter = 2 × (12 + 15) = 54 ft; gross wall = 54 × 9 = 486 sq ft. Subtract one door (21) and two windows (24) → 441 sq ft. Two coats at a typical 350 sq ft per gallon: 441 × 2 ÷ 350 = 2.52 gallons.
| Condition | Adjustment |
|---|---|
| Rough or unpainted plaster | +10–20% paint (higher absorption) |
| Dramatic color change | Extra coat or tinted primer |
| Dark → light | Tinted primer instead of 3rd coat |
| Airless sprayer | +25% (overspray loss) |
Ceilings add area separately (floor area × coats) and trim needs its own calculation by linear feet. The Paint Cost Estimator computes area, coats and price together; the Room Area tool handles irregular floor plans first.
Perimeter × height minus doors and windows, times the number of coats, divided by the spread rate (usually ~350 sq ft per gallon). A 12×15 ft room with 9 ft ceilings and two coats needs about 2.5 gallons.
One coat of a quality paint over a similar color often looks fine; two coats give even coverage and durability. Over a dramatic color change, use a tinted primer plus two coats.
Typically 350–400 sq ft per coat on smooth interior walls. Rough surfaces, deep colors and sprayers reduce it — plan on the low end when in doubt.
Yes. A door is about 21 sq ft and a typical window about 12 — subtracting them keeps the estimate honest without materially changing the buy decision on small rooms.