Paint Calculator

How to measure a room for paint

Three measurements and a count of doors and windows are all you need.

Last updated September 20, 2026

You need three measurements and a quick count of doors and windows. Measure in feet and write each number down as you go.

You will need: a tape measure, a pencil and paper (or your phone), and a helper if your walls are long.
How to measure a room for paint Two drawings. On the left, a top view of a rectangular room with its length and width marked. On the right, a side view of one wall with the ceiling height marked, plus a door and a window that are subtracted from the wall area. Length Width Floor Top view Door Window Ceiling height Wall length Side view of one wall
Measure the length, the width and the ceiling height. Count the doors and windows, because they are subtracted from the wall area.

Step by step

  1. Measure the length and width. Run the tape along the floor from wall to wall. If the room is not a simple rectangle, see the section on odd shapes below.
  2. Measure the ceiling height. Measure from the floor to the ceiling. For a vaulted or sloped ceiling, use the average height of the wall.
  3. Count the doors and windows. The paint calculator subtracts 21 sq ft for a standard 3 × 7 ft door and 15 sq ft for a standard 3 × 5 ft window. If yours are much bigger, count them as more than one, or change the sizes under Advanced settings.
  4. Work out the wall area. Add the length and width, double the result, and multiply by the ceiling height. Then subtract the doors and windows.
  5. Enter the numbers in the calculator. It figures the paint for you and shows every step of the math.

Turn inches into feet

The calculator uses feet, and decimals are fine. Divide the inches by 12: 3 inches is 0.25 ft, 6 inches is 0.5 ft and 9 inches is 0.75 ft. So a room that is 12 ft 6 in long is 12.5 ft.

Example: a room measured in feet and inches

Say the room is 12 ft 6 in by 10 ft 3 in, with 8-foot ceilings, one door and two windows.

StepMathResult
Length and width in feet12.5 and 10.2512.5 × 10.25 ft
Wall area2 × (12.5 + 10.25) × 8364 sq ft
Doors and windows1 × 21 + 2 × 1551 sq ft
Paintable area364 − 51313 sq ft
Wall paint to buyTwo coats, 350 sq ft per gallon, 10% extra2 gal

Odd shapes and single walls

  • L-shaped rooms. Add up the length of every wall you will paint and multiply by the ceiling height. In the calculator, enter any length and width that add up to half of that total wall length. This works for walls only, so figure the ceiling from its own area.
  • An accent wall. Multiply the wall's width by the ceiling height, then subtract any door or window on it. Divide by the coverage of your paint and multiply by the number of coats.
  • Alcoves and bump-outs. Measure each extra wall and add it to the total length of wall.

Mistakes to avoid

  • Entering inches where the calculator expects feet.
  • Forgetting to subtract doors and windows, or subtracting windows you plan to paint around.
  • Leaving out the ceiling when you plan to paint it. Tick "Paint the ceiling too" in the calculator.
  • Rounding down. It is safer to round measurements up.
Ready with your numbers? Go to the paint calculator.

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