Paint Calculator

Exact wall area, the paint the job really needs, and a sensible shopping answer in gallons and quarts — or liters.

The space

Buy

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Wall area—
Area × coats—
Exact paint—
Per coat—
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Assumptions you can change

Standard deductions: 21 ft² (1.95 m²) per door and 15 ft² (1.4 m²) per average window. Coverage defaults to 350 ft²/gal (≈8.6 m²/L) — typical for quality paint on smooth, primed walls. Rough, porous or dark-to-light surfaces cover 25–30% less; drop the coverage number accordingly. The shopping answer rounds up to gallons and quarts (or half-liters) because that's how paint is sold.

How the math works

Wall area = perimeter × height, minus standard openings: 2 × (length + width) × height − doors × 21 − windows × 15 (ft²). Multiply by coats, divide by coverage, and round up to purchasable containers. Two coats is the honest default — one-coat claims survive only on similar colors over smooth walls.

When to adjust coverage

New drywall and fresh plaster drink paint (−25%). Textured or masonry surfaces too. Painting light over dark? Either budget a third coat or use a tinted primer — primer is cheaper per gallon than good paint. Cheap paint with low solids covers less per gallon than the label promises; the 350 default assumes a mid-to-premium product.

Don't skip the leftover plan

Deliberately keep a quart for touch-ups — walls get scuffed, and batch colors drift between production runs. This is why the calculator shows quarts instead of blindly rounding everything to whole gallons.

Frequently asked questions

How much does a gallon of paint cover?
About 350–400 ft² per coat on smooth, primed walls; the calculator defaults to 350. Rough, porous or previously unpainted surfaces cover 25–30% less — lower the coverage field for those.
How many coats do I need?
Two, in almost all real cases. One coat only works with similar colors on smooth walls; going light over dark may need three, or two over a tinted primer.
What deductions are used for doors and windows?
The trade standards: 21 ft² per door and 15 ft² per average window. Big picture windows or patio doors are worth measuring and entering via the direct-area mode instead.
Should I subtract the ceiling from wall height?
No — measure floor to ceiling for the walls. Tick the ceiling option to add length × width of ceiling area, usually painted a different product (flat ceiling paint).
How much paint for a 12×12 room?
With 8-ft walls, one door and two windows: 2×(12+12)×8 − 21 − 30 = 333 ft²; two coats = 666 ft² ÷ 350 ≈ 1.9 gallons — buy two.

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