Paint Calculator — How Much Paint Do I Need?
Calculate gallons of paint needed for a room. Enter dimensions, count doors and windows, and pick the number of coats — get gallons needed plus estimated cost.
Standard door = 21 sq ft subtracted from paintable area.
Standard window = 12 sq ft subtracted from paintable area.
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Frequently asked questions
How much area does one gallon of paint cover?
Standard interior latex paint covers about 350 sq ft per gallon on a smooth, primed surface. Heavy-bodied premium paints (Behr Marquee, Sherwin-Williams Emerald) advertise 400 sq ft per gallon. Textured walls and ceilings drop coverage to ~250 sq ft. Exterior paint over rough siding can be as low as 200 sq ft per gallon. Always read the back of the can — manufacturers print actual coverage.
Do I need primer?
Yes for: bare drywall, bare wood, dramatic color changes (light over dark or vice versa), stained surfaces, glossy old paint. No primer needed for: same-color refresh on existing latex paint, or when using paint-and-primer-in-one products on lightly tinted existing paint. When unsure, prime — primer is cheaper than a third topcoat.
How do I calculate ceiling paint separately?
Ceiling area equals the floor area: length × width of the room. A 12×10 room has 120 sq ft of ceiling, requiring 1 gallon for 2 coats (240 sq ft of coverage needed, well under one gallon's 700 sq ft for 2-coat coverage). Most rooms only need 1 gallon of ceiling paint. Use a flat-finish ceiling paint, not the same paint as the walls.