Paint Calculator
Enter the room's dimensions, openings and coats to get gallons and quarts to buy. The math uses the industry-standard spread rate (350 sq ft per gallon, editable to match your paint's label) and shows itself step by step.
Paint math
Wall area is the room's perimeter times wall height (add the ceiling's length × width if you're painting it). Subtract standard openings — about 21 sq ft per door and 15 per window — multiply by coats, and divide by the paint's spread rate. One gallon covers about 350 sq ft per coat; the real number is printed on your can (250–400 depending on the paint and surface).
Plan two coats for color changes and new drywall; porous or textured surfaces drink more (use the low end of the spread rate). Buy rounded up to the quart — and keep the leftover labeled for touch-ups.
Worked examples
Example 1: a 12 × 10 bedroom, two coats
Walls 8 ft high, one door, two windows.
- Wall area.2 × (12 + 10) × 8 = 352 sq ft
- Subtract openings.352 − 21 − 30 = 301 sq ft
- Two coats at 350 sq ft/gal.301 × 2 ÷ 350 = 1.72 gal
Buy 1 gallon + 3 quarts (or round to 2 gallons for touch-up reserve).
Example 2: just the ceiling
A 12 × 10 ceiling, one coat of ceiling paint.
- Ceiling area.12 × 10 = 120 sq ft
- One coat.120 ÷ 350 = 0.34 gal
Two quarts cover it (0.34 gal → round up to 2 qt).
Example 3: accent wall math
One 14 ft wall, 9 ft high, two coats of deep color.
- Single-wall area.14 × 9 = 126 sq ft
- Deep colors often need 2–3 coats; take 3 to be safe.126 × 3 ÷ 350 = 1.08 gal
A gallon plus a quart — dark accents eat more coats than people expect.
FAQ
How much does a gallon of paint really cover?+
Manufacturers rate 350–400 sq ft per gallon on smooth, primed surfaces; textured, porous or unprimed walls drop that to 250–300. Use the number on your can — this calculator's coverage field is editable for exactly that reason.
Do I need two coats?+
Usually yes: color changes, new drywall and budget paints all want two. One coat works only when repainting the same color with quality paint. Drastic changes (deep red over white) can take three.
Should I subtract doors and windows?+
Yes — standard figures are ~21 sq ft per door and ~15 per window, which this calculator uses. Skip the subtraction only if you have huge glass walls to be safe on the margin.
Does primer count in the gallons?+
No — primer is a separate product with its own coverage (usually ~300 sq ft/gal). Run this calculator once for primer (1 coat) and once for paint if the surface needs priming.
Gallon vs quart economics?+
A quart typically costs 40–60% of a gallon's price for a quarter of the paint — if you need 3+ quarts, the gallon is almost always the better buy and leaves touch-up reserve.
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