One gallon covers about 350 square feet per coat on smooth, previously painted drywall. Most rooms need two coats, so the working formula is:
gallons = (paintable_area × coats) ÷ 350Measuring paintable area
- Perimeter — add up the wall lengths. A 12 × 14 room is (12 + 14) × 2 = 52 ft.
- × wall height — 52 × 8 = 416 sq ft of gross wall.
- Subtract openings — about 21 sq ft per door, 15 sq ft per average window.
- × coats, then ÷ 350.
For that room with one door and two windows: 416 − 21 − 30 = 365 sq ft. Two coats is 730 sq ft, or 2.1 gallons — buy two gallons and a quart, or just two if you’re a careful cutter.
The Paint Calculator does the deductions and the coat multiplication, and tells you whether to buy gallons or quarts.
Gallons by room size (walls, two coats, 8 ft ceilings)
| Room size | Paintable wall area | Gallons for 2 coats |
|---|---|---|
| 10 × 10 ft | ~284 sq ft | 2 |
| 12 × 12 ft | ~348 sq ft | 2 |
| 12 × 16 ft | ~412 sq ft | 2.5 |
| 15 × 20 ft | ~524 sq ft | 3 |
| 20 × 20 ft | ~604 sq ft | 3.5 |
Assumes one door and one window deducted. Add roughly 25% for 10 ft ceilings.
When 350 sq ft per gallon is wrong
The number on the can is a best case — smooth, sealed, similar-colored. Real coverage drops with:
- Texture. Knockdown or orange peel can cost 10–20%; heavy texture and popcorn far more.
- Porosity. New drywall, patched areas, and bare wood drink paint. Prime first — primer is cheaper than a third coat of finish.
- Color change. Going light over dark, or covering a saturated red or deep blue, can need three coats. A tinted primer usually saves one.
- Application. Spraying uses noticeably more paint than rolling, even with good technique.
Check the spread rate printed on the actual can and adjust the coverage figure in the calculator to match.
Ceilings, trim, and doors
- Ceiling: length × width, so a 12 × 14 room is 168 sq ft — about half a gallon per coat. Use a flat ceiling paint, and buy it separately from the wall color.
- Trim: baseboard, casing, and crown usually come in well under a gallon for a single room. A quart of enamel covers a lot of trim.
- Doors: roughly 1 quart per side per two doors, depending on style and sheen.
Do I really need two coats?
Plan on two. One coat only works when the new color is close to the old one and the surface is in good shape. Two coats also wear better — the film thickness is what makes paint scrubbable, which matters in kitchens, baths, and hallways.
Buying tips
- Round up to the next container, but check the price break: two gallons and two quarts often costs more than three gallons.
- One batch, one room. Paint is tinted per batch and tiny color differences show on a long wall. If you might need a fifth gallon, buy it now — or box the paint (mix all cans in one bucket) before you start.
- Keep the leftovers labeled with the room and the color formula from the can lid. Touch-ups are inevitable.
Measure your room once and put the numbers into the Paint Calculator — it handles the openings, the coats, and the gallon-versus-quart decision.