Paint Calculator: Gallons for Walls & Ceilings

Estimate gallons of paint for a room from wall size, doors, windows and coats.

✓ Formula shown✓ No signupUpdated 2026-10-22

Room perimeter (ft) Wall height (ft) Doors Windows Coats Coverage (sq ft/gal)

Before you buy

Most people under-order. Add the waste allowance shown above, and buy one extra bag or box for touch-ups. Compare current prices for paint:

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Worked examples

Room	Paintable area	Paint
40 ft perimeter, 8 ft walls	269 sq ft	1.5 gal (2 coats)
48 ft perimeter, 8 ft walls	333 sq ft	1.9 gal (2 coats)
60 ft perimeter, 9 ft walls	489 sq ft	2.8 gal (2 coats)
80 ft perimeter, 8 ft walls	589 sq ft	3.4 gal (2 coats)

How the calculation works (sources & assumptions)

Paintable area = perimeter × height − 21 sq ft per door − 15 sq ft per window (common allowances). Gallons = area × coats ÷ coverage. Most labels state 250–400 sq ft per gallon.

FAQ

How much paint for a 12x12 room?

The perimeter is 48 ft; with 8 ft walls, 1 door and 2 windows, that is about 333 sq ft, or roughly 2 gallons for two coats.

Do I need primer?

Use primer for bare drywall, stains or dramatic color changes. Self-priming paints reduce, but don't always remove, the need for it.

Paint Calculator: Gallons for Walls & Ceilings

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