Paint Calculator

How many gallons of wall paint you need.

How it’s calculated

Paintable area = wall length × wall height − (doors × 21 sq ft) − (windows × 15 sq ft).

Gallons = area × coats ÷ coverage × (1 + waste%).

Deductions assume a 3×7 ft door and 3×5 ft window. Coverage: Behr lists 250–400 sq ft per gallon “depending on the application method and substrate porosity” and notes this excludes spray loss (Behr product page). We default to 350; use 250 for bare drywall, texture or strong color changes, and check your can’s label.

Worked examples

12×12 ft bedroom, 8 ft ceilings: perimeter 48 ft × 8 = 384 sq ft; minus 1 door (21) and 2 windows (30) = 333 sq ft. Two coats = 666 sq ft ÷ 350 = 1.90 gal; +10% = 2.09 gal → buy 3 gallons (or 2 gallons + 1 quart).

Same room at 400 sq ft/gal, no waste: 666 ÷ 400 = 1.67 → 2 gallons.

Waste factor

10% is a common cushion for roller loss, touch-ups and a little left for later repairs. Spraying wastes far more and is not included in Behr’s coverage figures — add 20–30% if you spray (our estimate).

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