Paint is sold by coverage, and the number you need comes down to one ratio: a gallon of interior wall paint covers roughly 350–400 square feet in a single coat. Most jobs need two coats, and most walls have doors and windows you don’t paint — so the real question is how much paintable surface you have.
How much paint do I need?
Measure the perimeter of the room (add up the length of every wall) and multiply by the wall height to get gross wall area. Subtract about 21 sq ft for each standard door and 15 sq ft for each average window, then divide by 350 and multiply by the number of coats.
Worked example
A 12 ft × 14 ft room with 8 ft ceilings has a perimeter of (12 + 14) × 2 = 52 ft. Times 8 ft height = 416 sq ft. Subtract one door (21) and two windows (30) → 365 sq ft of paintable wall. At 350 sq ft per gallon, that’s about 1 gallon per coat, or 2 gallons for two coats. Painting the 12 × 14 ceiling too (168 sq ft) adds roughly another gallon for two coats.
Paint coverage chart
A quick estimate of gallons for two coats on walls, before subtracting large openings:
| Room size | Approx. wall area | Gallons (2 coats) |
|---|---|---|
| 10 × 10 | ~300 sq ft | 2 |
| 12 × 14 | ~365 sq ft | 2 |
| 15 × 20 | ~520 sq ft | 3 |
| Avg bedroom + ceiling | — | 3–4 |
Do I need primer?
Buy primer separately if you’re covering bare drywall, going from a dark colour to a light one, or painting over stains. Primer coverage runs a bit lower — about 200–300 sq ft per gallon. Textured or porous walls drink more paint, so round up.
Frequently asked questions
How many square feet does a gallon of paint cover? About 350–400 sq ft for one coat on a smooth, primed wall. Use 350 to be safe.
Do I really need two coats? Usually yes — one coat rarely gives even colour, especially over a different shade. Same-colour touch-ups can be the exception.
Should I include the ceiling? Only if you’re painting it. Calculate it separately as length × width.