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.
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.
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.
Related calculators
Note: Estimates vary with surface texture, paint brand, and application method. Check the coverage on your paint can. Last reviewed: July 2026.