Paint Calculator
Estimate the gallons of paint a room needs from its wall dimensions, the number of coats, and the doors and windows you'll skip.
How paint quantity is calculated
Gallons = (wall area × coats) ÷ coverage per gallon
We subtract about 20 sq ft per door and 15 sq ft per window as typical openings.
A 12×12 room, 8 ft walls: perimeter 48 × 8 = 384 sq ft, minus 1 door (20) and 2 windows (30) = 334 sq ft. Two coats ÷ 350 coverage = ~1.9 gallons → buy 2.
One 14 ft × 9 ft wall, no doors or windows, one coat: 14 × 9 = 126 sq ft ÷ 350 sq ft/gal ≈ 0.36 gallon — round up to one gallon, even though well under half the can is actually used.
Getting the estimate right
Buy in full gallons and round up — the last stretch of wall is the worst time to run out, and matching a later batch is risky. Two coats is standard for an even finish; big color changes (dark to light especially) may need primer plus two coats. Textured or porous surfaces cover less than the 350 sq ft default, so drop the coverage number for those. Ceilings are a separate area (length × width) — run them through the square footage calculator and add a coat's worth. Trim and doors use much less and are usually a separate can.
Paint coverage by surface type
| Surface | Coverage per gallon (1 coat) |
|---|---|
| Smooth drywall (primed) | 350–400 sq ft |
| Textured / orange-peel drywall | 300–350 sq ft |
| Stucco or heavy texture | 200–250 sq ft |
| Brick (unpainted, porous) | 100–150 sq ft |
| Previously painted wall, same color | 400+ sq ft |
Common paint estimating mistakes
- Forgetting ceilings and trim need separate paint totals. This calculator only covers the four walls; a ceiling is length × width (not perimeter × height) and typically needs a different, flatter sheen, and trim/doors use a separate, smaller can.
- Assuming leftover paint from a similar past project will match exactly. Even the same "name" of color can shift slightly between manufacturing batches — bring a chip or can label to the store for an exact re-match rather than guessing.
- Buying the exact calculated amount with nothing extra. Running out mid-wall on the second coat, especially with a custom-tinted color, means a delay while a new batch is mixed and a risk of a visible tint difference — a full extra quart or rounding up to the next gallon avoids this.
Frequently asked questions
How much paint do I need for a room?
Wall area minus openings, ÷ ~350 sq ft/gallon, × coats. A 12×12 room ≈ 2 gallons for two coats.
How much does a gallon cover?
About 350–400 sq ft per coat on smooth walls; less on rough surfaces.
One coat or two?
Usually two; big color changes may need primer plus two.
How much paint do I need for just one accent wall?
Use that wall's own width × height, subtract openings, ÷ coverage per gallon. A 14×9 ft wall is 126 sq ft — well under half a gallon for one coat.
Does a darker color need more paint?
Often indirectly — big color changes may need a primer coat or an extra coat, which affects total gallons more than coverage per gallon.
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Note: Estimates vary with surface texture, paint brand, and application method. Check the coverage on your paint can. Last reviewed: September 2026.