Example: a 14 × 12 ft bedroom with 8 ft walls
- Wall area: 2 × (14 + 12) × 8 = 416 sq ft.
- Subtract 1 door (20 sq ft) and 2 windows (2 × 15 sq ft): 416 − 50 = 366 sq ft.
- Two coats: 366 × 2 = 732 sq ft of coverage.
- At 350 sq ft per gallon: 732 ÷ 350 = 2.1 gallons — buy 2 gallons + 1 quart.
How it's calculated
- Wall area = room perimeter × wall height, where the perimeter is 2 × (length + width). For single walls, use width × height.
- Subtract about 20 sq ft (1.9 m²) per door and 15 sq ft (1.4 m²) per window.
- Add the ceiling (length × width) if you're painting it.
- Paint = area × number of coats ÷ coverage per gallon (or liter).
- Round up to real cans: quarts for small leftovers, gallons, and 5-gallon buckets for big jobs.
Assumptions
- Coverage defaults to 350 sq ft per gallon (10 m² per liter), realistic for most wall paints on smooth, previously painted walls. Change it under Options to match your can.
- Rough, textured, porous or unprimed surfaces soak up more paint — lower the coverage setting for them.
- Can sizes: quarts, gallons and 5-gallon buckets (US), or 1, 2.5, 5 and 10 L tins. We pick the combination that covers the job at the lowest typical price.
- Trim, doors and cabinets usually get a different paint and aren't included.
Frequently asked questions
How much does a gallon of paint cover?
Most interior wall paints cover about 350–400 sq ft per coat on smooth, previously painted walls. Textured, porous or bare surfaces can drop to 250 sq ft or less. The can lists its coverage — enter it under Options for the most accurate answer.
Do I need one coat of paint or two?
Plan for two. Two coats give an even color and a tougher finish, especially when you're changing colors. One coat can be enough when you're refreshing the same color over a clean, sound surface.
How much paint do I need for a 12 × 12 room?
With 8 ft walls, a 12 × 12 room has 384 sq ft of wall. Subtract one door and one window (about 35 sq ft) and you have 349 sq ft. Two coats at 350 sq ft per gallon come to about 2 gallons.
Should I subtract doors and windows?
Yes — they add up. We subtract about 20 sq ft per door and 15 sq ft per window. Patio doors and picture windows are much bigger, so measure them and use Wall(s) mode if you want to be precise.
Do I need primer?
Use primer on new drywall, bare wood, patched spots and stains, or when covering a dark color with a light one. Estimate it the same way: pick 1 coat and enter the coverage printed on the primer can.
Can I use the same paint on the ceiling?
Usually not — ceilings are typically painted with flat ceiling paint that hides imperfections. When you include the ceiling, the calculator lists walls and ceiling separately so you can buy each paint.