Paint coverage calculator

Enter the total length of the walls and their height to see how many gallons a room takes, with doors, windows and a second coat accounted for.

Add up all the walls you're painting. A 12 × 12 room is 48 ft.

Most interior paints cover 350 to 400 sq ft per gallon on smooth walls; check the can.

  1. Wall areasquare feet384.00
  2. Less 1 doors and 2 windows-51.00
  3. Paintable area333.00
  4. Area for 2 coats666.00
  5. Gallons needed at 350 sq ft per gallon1.903
  6. Or, in quarts8
Buy this many gallons 2

Last updated 13 Sep 2026.

How to use it

  1. Measure each wall you're painting and add the lengths together.
  2. Enter the ceiling height, and count the doors and windows so their area comes off.
  3. Set the number of coats. Two is normal; three for a big colour change or over a dark wall.
  4. Read the gallons to buy. The exact figure is below it if you're mixing gallons and quarts.

Worked example

A 12 × 12 ft room with 8 ft ceilings, one door and two windows, two coats of paint covering 350 sq ft per gallon.

Walls total 48 × 8 = 384 sq ft, less 21 for the door and 30 for the windows = 333 sq ft. Two coats is 666 sq ft ÷ 350 = 1.903 gallons, so buy 2 gallons.

How it's calculated

wall area = total wall length × height

paintable area = wall area − doors × 21 sq ft − windows × 15 sq ft

gallons = paintable area × coats ÷ coverage per gallon

cans = gallons rounded up to the next whole can. Ceilings and trim aren’t included; run them separately.

FAQ

Should I buy extra?

A little. Rounding up to whole cans usually leaves enough for touch-ups. Textured walls, bare drywall and rough plaster soak up more, so drop the coverage figure to 250 to 300 sq ft per gallon for those.

How much does a ceiling take?

Length × width ÷ coverage, times coats. A 12 × 12 ceiling is 144 sq ft, so one coat is under half a gallon. Ceiling paint is usually one or two coats.

Do I need primer?

On bare drywall, patched areas, glossy surfaces or when going from dark to light, yes, and it needs its own coat's worth of product. Paint-and-primer-in-one products still usually need two coats.

Is it cheaper to buy a 5-gallon bucket?

Per gallon, yes, typically 15% to 25% less. It makes sense from about 4 gallons up, or when several rooms take the same colour.