Paint Calculator

How paint coverage is calculated

The math is: measure the walls, subtract what you won't paint, multiply by the number of coats, and divide by how much area a gallon covers.

Wall area = 2 × (length + width) × ceiling height
Paint needed = (wall area − doors & windows + ceiling if included) × coats ÷ coverage per gallon

This calculator deducts the standard sizes painters use: about 21 sq ft per door (a standard 3 ft × 7 ft door) and 15 sq ft per window. If your room has oversized windows, a wall of built-ins, or a big open doorway to another room, add or subtract a "window" or "door" to compensate.

How far does a gallon of paint go? Most manufacturers rate a gallon at 350–400 sq ft on smooth, primed walls — 350 is the safe planning number used above. Rough, textured, or unprimed drywall drinks more paint (plan on 300 or less), and a dramatic color change may need an extra coat or a tinted primer first.

Do I need primer too? On new drywall, bare wood, or over stains and strong colors, yes — primer is roughly one coat at the same coverage, and it usually makes the finish paint cover in fewer coats. Paint-and-primer-in-one products still benefit from a dedicated primer on raw surfaces.

Should I round up? Yes — the "paint to buy" figure above already rounds up to whole gallons. Keeping a little extra of the same paint (and noting the color code) makes touch-ups invisible later; an exact-to-the-drop estimate leaves you short mid-wall.