How to work out how much paint a room needs
Paint is sold by coverage, not by floor size, so the math runs on wall area. Add up the length of every wall (the perimeter), multiply by the wall height, subtract the doors and windows, then multiply by the number of coats and divide by the paint's spread rate:
gallons = (perimeter × height − doors − windows) × coats ÷ sq ft per gallon
Worked example — a 12×12 bedroom with 8-ft walls, one door and two windows: 48 ft of wall × 8 ft = 384 sq ft. A standard 3×7 door is 21 sq ft and each 3×5 window is 15 sq ft, so the paintable area is 384 − 21 − 30 = 333 sq ft. Two coats at 350 sq ft per gallon: 333 × 2 ÷ 350 = 1.9 gallons. Buy 2 gal.
Paint for common room sizes
| Room (8-ft walls) | Wall area | Two coats | Buy |
|---|---|---|---|
| 10×10 room | 284 sq ft | 1.62 gal | 2 gal |
| 10×12 room | 301 sq ft | 1.72 gal | 2 gal |
| 12×12 room | 333 sq ft | 1.9 gal | 2 gal |
| 12×14 bedroom | 365 sq ft | 2.09 gal | 2 gal + 1 qt |
| 12×16 room | 376 sq ft | 2.15 gal | 2 gal + 1 qt |
| 14×16 room | 393 sq ft | 2.25 gal | 2 gal + 1 qt |
| 15×20 living room | 473 sq ft | 2.7 gal | 3 gal |
| 5×8 bathroom | 187 sq ft | 1.07 gal | 1 gal + 1 qt |
Each row deducts that room's doors and windows. A 9-ft ceiling adds 12.5% more wall; a 10-ft ceiling adds 25%.
How far does a gallon go?
Most premium interior paints are rated at 350–400 sq ft per gallon per coat on smooth, primed walls — and less on porous or textured surfaces. We plan with the low end of each maker's range so you don't run short. A quart is a quarter of that: about 87–100 sq ft per coat.
| Paint | Label coverage (sq ft/gal) | Coats |
|---|---|---|
| Sherwin-Williams Emerald Interior | 350–400 | 2 coats; self-primes drywall with 2 coats |
| Sherwin-Williams SuperPaint Interior | 350–400 | Primer + 2 coats on new surfaces |
| Benjamin Moore Regal Select | 400–450 | 2 coats on new/unprimed, 1–2 over paint |
| Benjamin Moore ben Interior | 400–450 | 1–2 coats, self-priming |
| Behr Ultra Scuff Defense | 250–400 | 2 coats; tinted primer for drastic color changes |
| Valspar Signature | 200–400 | 2 coats |
| Glidden (PPG) High Endurance Plus | up to 400 | 2 coats on bare surfaces |
| Clark+Kensington Interior Paint + Primer | 400–450 | 1–2 coats |
How to measure
- Measure each wall at floor level and add them up; for an L-shaped room, add every wall segment.
- Measure height from floor to ceiling — not to the top of the trim.
- Sherwin-Williams' calculator deducts 21 sq ft per door and 15 per window; Benjamin Moore's uses 21 and 12. Oversized patio doors and picture windows deserve their real size.
- Subtract wall hidden by kitchen cabinets, backsplash or tile — measure it; in a kitchen it can be a big share of the wall.
Coats and primer
Two coats is the norm for an even color. A premium line can cover in one coat when you repaint the same or a similar color. New drywall needs a coat of primer first — bare joint compound soaks up paint unevenly. Drywall primers cover 250–400 sq ft per gallon on bare board. For a big color change, a primer tinted gray or toward the new color usually saves a third topcoat; for water stains, spot-prime with a stain-blocking primer — a quart is usually enough.
Ceilings and trim
A ceiling equals the floor area (12×12 = 144 sq ft), so two coats take about 0.8 gallon. Popcorn and heavy textures drink more — we plan them at 75% of the label rate, about a third more paint (an estimate). For trim, multiply the linear feet by the face height: 48 ft of 6-in baseboard is 24 sq ft, so a quart of semi-gloss covers baseboards and casings in most rooms.
Common mistakes
- Using the floor area instead of the wall area — a 12×12 room has 144 sq ft of floor but about 333 sq ft of wall.
- Buying three quarts instead of a gallon: a quart usually costs 36–52% of a gallon, so three cost more than one gallon.
- Mixing cans from different batches on one wall — box (combine) your paint when you buy four or more gallons.
- Forgetting that sprayers lose paint to overspray: plan about 25% more than with a roller.
- Sanding old paint in a home built before 1978 without testing for lead first.
Painting outside too? Use the exterior paint calculator or the deck stain calculator. Replacing trim or floors at the same time? Try the baseboard calculator.