How this is worked out
The walls are the room's perimeter times its height, and the ceiling is the length times the width. Each door takes off 20 sq ft and each window 12, the size of a 3' × 6' 8" door and a 3' × 4' window. Multiply what's left by the coats, divide by the coverage on the can, and round up to whole gallons. No waste is added: the coverage figure already allows for normal use.
That rounding is why a 12 × 12 bedroom with a door and a window needs three gallons for two coats rather than two: 704 sq ft is just over what two gallons cover. Every length is rounded to the nearest sixteenth first, the way Handyman Calc does it.
For common rooms already worked out, there's a paint gallons chart for 8 and 9 ft walls, with a PDF to print.
On your phone, on the job site
On a phone, Handyman Calc has this paint screen, where each opening can be measured, and fourteen more calculators. It works with no signal, and it's free.
▶ Try it in your browser Download on the App Store ▶ Get it on Google Play
Charts worked out with the same math:
- how many bags of concrete a slab takes
- how many sheets of drywall a room takes
- a fraction to decimal chart for inches
- how much paint a room takes
- how much mulch a bed takes
- a roof pitch chart
- how many steps a rise takes
- the diagonal that squares a frame
- board feet for common lumber
More calculators:
Frequently asked questions
How many gallons of paint for a 12x12 room?
3 gallons for two coats on the walls of a 12 × 12 room with 8 ft walls, one door and one window. The walls come to 352 sq ft, so two coats is 704 sq ft, just over two gallons at 350 sq ft a gallon.
How many gallons of paint for a 10x12 room?
2 gallons for two coats on the walls, with 8 ft walls, one door and one window: 320 sq ft, or 640 sq ft for both coats. Add the ceiling and it's 3.
How much does a gallon of paint cover?
About 350 sq ft for one coat on smooth, primed drywall, which is what the calculator uses unless you change it. Rough or porous surfaces take more. Check the can: most give 250 to 400 sq ft.
How do you calculate how much paint you need?
Double the length plus the width, multiply by the wall height, and take off the doors and windows. Add the ceiling if you're painting it. Multiply by the number of coats and divide by the coverage on the can. Round up to whole gallons.
Do I need two coats of paint?
Usually. A color change, a new wall or a cheap paint needs two. A fresh coat of the same color over sound paint can get away with one. The calculator starts at two.
Should I buy quarts or a gallon?
Buy quarts only for small jobs. Four quarts usually cost more than a gallon, so the calculator mentions quarts only when the job needs a gallon or less.