Paint Calculator — How Much Paint Do I Need?
Enter your room's dimensions and the number of doors and windows to get the exact gallons of paint you need for the walls. Adjust coats and coverage to match your project. No sign-up, no data stored.
Paint needed (walls)
How paint amount is calculated
The calculator uses the standard formula painters use:
- Wall area = room perimeter × height = (length + width) × 2 × height.
- Subtract openings: a standard door ≈ 20 sq ft, a typical window ≈ 15 sq ft.
- Net area = wall area − (doors × 20) − (windows × 15).
- Gallons = net area × coats ÷ coverage per gallon, rounded up to whole gallons.
Coverage varies by surface. Typical latex paints list 350–400 sq ft per gallon for one coat on smooth walls; we default to 350 to be safe. Rough or unprimed surfaces cover less.
Paint coverage & sizing reference
Standard values for planning without the calculator.
Coverage by surface
| Surface | Coverage / gallon |
|---|---|
| Smooth, well-primed wall (latex) | 350–400 sq ft |
| Rough / textured wall | 200–300 sq ft |
| Primer | ~300 sq ft |
| Unprimed drywall | 250–300 sq ft |
Common room sizes → gallons (2 coats)
| Room (ft × ft, 8 ft ceiling) | Walls after door/window | Gallons (2 coats) |
|---|---|---|
| 10 × 10 (small bedroom) | ~285 sq ft | 2 |
| 12 × 10 | ~317 sq ft | 2 |
| 12 × 12 | ~349 sq ft | 2 |
| 14 × 16 | ~429 sq ft | 3 |
| 16 × 20 (living room) | ~501 sq ft | 3 |
Last updated: October 2026 · Coverage values are typical manufacturer ranges (Sherwin-Williams, Behr, etc.). Door ≈ 20 sq ft, window ≈ 15 sq ft are standard US sizes.
Paint calculator FAQ
A gallon of typical latex paint covers about 350–400 sq ft per coat. We default to 350 to avoid running short.
A 12×12 room with 8 ft ceilings has ~384 sq ft of walls. After one door and one window (~320 sq ft net), you need about 1 gallon for one coat, or 2 gallons for two coats.
Yes — a door is about 20 sq ft and a window about 15 sq ft. Subtracting them avoids overbuying.
Primer covers ~300 sq ft/gal and is recommended on bare drywall, stained walls, or when making a big color change. One coat of primer then two coats of finish is the standard.
Buying one extra quart (or a gallon if you may patch later) is smart for touch-ups and mistakes. Store paint with the lid sealed tight.
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