A gallon of paint covers 350–400 square feet on a smooth, primed surface. For a whole house, the fast estimate is floor area × 2.5 for interior wall surface, divided by 350, times the number of coats.
For a 1,200 sq ft house: 1,200 × 2.5 = 3,000 sq ft of wall, or about 8.5 gallons per coat — call it 17 gallons for two coats of the whole interior. Most people don’t paint every room at once, so the per-room number below is usually the one you want.
Interior gallons by house size
Walls only, two coats, using floor area × 2.5 as the wall estimate.
| House size | Wall area | Gallons (2 coats) |
|---|---|---|
| 800 sq ft | 2,000 sq ft | 12 |
| 1,000 sq ft | 2,500 sq ft | 15 |
| 1,200 sq ft | 3,000 sq ft | 17 |
| 1,500 sq ft | 3,750 sq ft | 22 |
| 2,000 sq ft | 5,000 sq ft | 29 |
| 2,500 sq ft | 6,250 sq ft | 36 |
Ceilings add roughly one gallon per 350 sq ft of floor area per coat. Trim is usually a quart to a gallon per room, in a different product and sheen.
The ×2.5 multiplier is a planning shortcut that assumes 8 ft ceilings and an average number of interior walls. Measure room by room if you can — an open-plan house has far less wall per square foot than a chopped-up one, and the multiplier can be off by a third either way.
The accurate way: room by room
For each room: 2 × (length + width) × ceiling height, then subtract about 21 sq ft per door and 15 sq ft per window. Divide by 350 and multiply by your coat count.
A 12 × 14 bedroom with 8 ft ceilings, one door and two windows:
- Walls: 2 × 26 × 8 = 416 sq ft
- Less openings: 416 − 21 − 30 = 365 sq ft
- One coat: 365 ÷ 350 = 1.04 gallons
- Two coats: 2.1 gallons — buy 2 gallons and a quart, or just 3
Exterior is a different calculation
Exterior paint goes further on the label and much less far in practice. Two adjustments matter:
- Coverage drops with texture. Smooth siding gets the full 350–400 sq ft per gallon. Rough cedar, stucco, and split-face block can drop to 200 sq ft or less — plan on 50% more paint than the label suggests.
- Estimate the wall, not the floor. Measure each elevation: width × height, plus roughly half of the gable triangles. A 1,200 sq ft single-storey house is typically 1,100–1,400 sq ft of exterior wall — 7–8 gallons for two coats on smooth siding, 11–14 on rough.
Bare or previously unpainted exterior also needs primer, which is another full coat’s worth.
Does it really need two coats?
Plan on two. One coat only works when the new colour is close to the old one and the surface is sound. Two coats also wear better, because film thickness is what makes paint scrubbable — which is what matters in kitchens, baths, and hallways.
Dark colours over light, or any colour over bare drywall, may want a tinted primer plus two finish coats.
Buying notes
- Check the price break. Three gallons often costs less than two gallons plus two quarts, and five-gallon buckets are usually the cheapest per gallon by a wide margin.
- Box your paint. Tint varies slightly between cans, and the difference shows across a long wall. Mix all your cans together in one bucket before you start.
- Buy the fifth gallon now, not next week. Batches change.
Measure a room and put the numbers into the Paint Calculator — it handles the openings, the coats, and the gallon-versus-quart decision for you.