TABLE READY
1. Gallons per room (walls only, 8 ft ceilings)
Wall area = perimeter × 8 ft, minus one door (20 sq ft) and one window (16 sq ft). Gallons = wall area × 2 coats × 1.1 ÷ 350 sq ft/gal, rounded up. The ceiling adds length × width × 2 × 1.1 ÷ 350 more.
| Room / project | Wall area | Walls, 2 coats | + Ceiling | Total buy |
|---|---|---|---|---|
| 10 × 10 ft room | 284 sq ft | 2 gal | +1 gal | 3 gal |
| 10 × 12 ft room | 316 sq ft | 2 gal | +1 gal | 3 gal |
| 12 × 12 ft room | 348 sq ft | 3 gal (2 + 1 qt) | +1 gal | 4 gal |
| 15 × 15 ft room | 444 sq ft | 3 gal | +2 gal | 5 gal |
| 15 × 20 ft living room | 524 sq ft | 4 gal | +2 gal | 6 gal |
| House 1,500 sq ft (walls + ceilings) | ≈ 2,850–3,300 sq ft | 17–19 gal | incl. | 4 five-gal pails |
| House 2,000 sq ft (walls + ceilings) | ≈ 3,800–4,400 sq ft | 24–28 gal | incl. | 5–6 five-gal pails |
Same math as the paint calculator (350 sq ft/gal per coat, 10% waste, rounded up to quarts). One coat over a similar color cuts roughly in half.
FORMULA
2. The formula for any room
Add up every wall's area, subtract doors and windows, multiply by coats and by 1.1 for waste, and divide by 350. The US paint calculator does exactly this and breaks the purchase into 5-gallon pails, gallons and quarts.
WHOLE HOUSE
3. Whole-house numbers
A house's wall area runs roughly 0.9–1.2× its floor area, plus the same again in ceilings. Plan in 5-gallon pails for anything over ~400 sq ft of wall: the pail price per gallon is lower and the math rounds cleanly.
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