How to use
- Choose Room size to calculate the four walls from length, width and ceiling height, or Wall area if you already know the area.
- Enter how many doors and windows there are and their size; they are subtracted from the wall area (defaults: doors 3 × 7 ft, windows 3 × 5 ft).
- Check the coats (usually 2), the coverage printed on the can, and the extra allowance.
- You get the gallons needed and how many quarts, gallons or 5-gallon buckets to buy. Switch to m · L for metric.
Formulas
| Step | Formula |
|---|---|
| Wall area | 2 × (length + width) × ceiling height |
| Area to paint | wall area (+ ceiling length × width) − doors and windows |
| Paint needed | area × coats ÷ coverage per gallon × (1 + extra%) |
| Cans | paint needed ÷ can size, rounded up |
Coverage is how much area one gallon (or liter) covers in a single coat. Interior latex paint is typically labeled at about 350–400 sq ft per gallon. A common rule of thumb is to subtract about 21 sq ft per door and 15 sq ft per window.
Example
A 12 × 10 ft bedroom with 8 ft ceilings, one door and two windows:
- Wall area: 2 × (12 + 10) × 8 = 352 sq ft
- Area to paint: 352 − 21 − 2 × 15 = 301 sq ft
- Two coats at 350 sq ft/gal: 301 × 2 ÷ 350 = 1.72 gal; with 10% extra, about 1.89 gal
- Buy 2 gallons — two 1-gallon cans beat one gallon plus four quarts.
Things to watch
- New drywall, bare plaster and dramatic color changes soak up more paint than the label suggests. Prime first or raise the extra allowance.
- Quarts cost more per gallon than gallons, and gallons more than 5-gallon buckets. Compare the least-leftover mix with simply buying a larger size.
- Trim, baseboards and door frames usually take a different paint and sheen, so estimate them separately.
FAQ
How many coats of paint do I need?
Two coats is the usual recommendation. A similar color over a sound surface may need only one; going from dark to light can take three, or primer plus two.
What about floors and backsplashes?
For tiled surfaces use the tile calculator, and for irregular areas work them out with the area calculator.