Paint Calculator

Paint needed depends on net wall area: perimeter × ceiling height, minus doors and windows, times the number of coats, divided by the manufacturer's coverage rate. One US gallon typically covers about 350 sq ft per coat on smooth, primed drywall (~8.6 m² per liter).

Rough texture, dark color changes, and porous surfaces (brick, stucco) reduce effective coverage — check the label or add 10–20% extra. Ceilings are often calculated separately if they use a different finish.

Example: a 12 ft × 12 ft room with 8 ft walls has 384 sq ft of gross wall (48 ft perimeter × 8). Subtract 60 sq ft of openings for 324 sq ft net. At 350 sq ft/gal and 2 coats, you need about 1.85 gallons — round up to 2.

Buy in the sizes that are actually sold. Paint comes in quarts and gallons, so a 1.85-gallon requirement means one gallon plus three quarts, or simply two gallons — and two gallons is often cheaper than a gallon plus quarts. Buying the full amount in one batch also keeps the colour consistent, since tint batches vary slightly.

Subtract openings only when they are worth subtracting. A standard interior door is about 21 sq ft and a typical window 12 to 15 sq ft, so a room with one door and one window loses roughly 35 sq ft of the total — under 10% of a small room's walls, which is close to the waste you should be holding back anyway.

Coverage claims on the label assume ideal conditions. The 350 sq ft per gallon figure describes a smooth, primed, previously painted surface rolled at the recommended film thickness; new drywall, bare wood, brick, and stucco can drop that to 200 to 250 sq ft, and spraying uses noticeably more paint than rolling.

Ceilings are usually calculated separately because they use a flat finish rather than the wall sheen. Ceiling area is simply the floor area — 144 sq ft in a 12 by 12 ft room — so one gallon covers two coats with room to spare.

Trim, doors, and baseboards are a different calculation again, priced by linear feet rather than wall area. A quart typically covers the trim of an average room, and trim usually needs two coats of a harder enamel.

Primer on dark or patched walls may require an extra coat not reflected in a single coverage pass — budget another gallon when color changes are dramatic or when covering stains with stain-block primer first.