Paint Calculator
Find out how much paint to buy before you are standing in the store. Add every room you are painting, set doors, windows and coats, and get the total paint to buy.
How it works
For each room:
- Wall area = (length + width) x 2 x wall height
- Subtract doors and windows using standard sizes
- Add the ceiling area (length x width) if you ticked it
- Paint needed = total area x coats / coverage per gallon or liter, rounded UP to whole cans
Worked example
A 12 ft x 10 ft room with 8 ft walls: perimeter 44 ft x 8 ft = 352 sq ft of wall. Minus 1 door (21) and 2 windows (30) leaves 301 sq ft. Add the 120 sq ft ceiling and you have 421 sq ft. Two coats is 842 sq ft of coverage, and at 350 sq ft per gallon that is 2.41 gallons - so buy 3 gallons, about $135 at $45 a gallon.
Assumptions to know
- Standard door and window sizes are subtracted - very large openings should be treated as extra "windows".
- Coverage defaults to 350 sq ft per gallon (about 10 sq m per liter), a typical mid-range paint.
- Primer, trim paint and textured-wall extra absorption are not included.
Results are planning estimates only. Always check the coverage and instructions on the actual product you buy, and measure your own space carefully before ordering. When in doubt, a little extra material is cheaper than a second trip or a stalled job.
Frequently asked questions
How much paint do I need for a 12 x 12 room?
Walls on a 12 x 12 ft room with 8 ft ceilings are about 384 sq ft before doors and windows. After a door and two windows, roughly 330 sq ft remains, so one coat needs about 1 gallon at typical coverage, and two coats need about 2 gallons.
Is one coat ever enough?
Sometimes, when painting a similar color over a primed wall. A color change, new drywall or a dark color usually needs primer plus two coats. Two coats is the safe default.
Does the calculator include primer?
No. If you need primer, run the same room numbers once for primer (usually 1 coat) and once for paint.
Why does coverage vary between paints?
Rough or porous walls soak up more paint, and deep colors cover less. The coverage printed on the can is the number to trust - edit the coverage field to match your can.