How to measure walls for paint
Measure a room's walls in feet, subtract doors and windows, and turn the square footage into gallons and quarts of paint.
- Updated October 10, 2026
- By the Paint Gallons editorial team
- 5 min read
The short answer
Measure the room's length, width and wall height in feet. Add the length and width, double the total, and multiply by the height: that is the wall area in square feet. Take off about 20 sq ft for each door and 12 sq ft for each window, multiply what is left by the number of coats, and divide by the coverage printed on the paint can, which is often 350 to 400 sq ft per gallon for interior latex paint. The paint calculator runs the same arithmetic and turns the answer into gallons and quarts.
This guide is about interior walls. Ceilings are measured as a flat rectangle instead, so the ceiling paint calculator handles them, and siding, trim and gables on the outside of a house belong in the exterior paint calculator.
Measure each wall
Run a tape measure along the floor from corner to corner for the length, then across the other way for the width, and measure the height from the floor to the ceiling. In a rectangular room the opposite walls match, so the distance all the way around the room, the perimeter, is 2 × (length + width). Multiply the perimeter by the wall height and you have the gross wall area, before anything is taken off for openings.
Take a bedroom 12 ft long and 10 ft wide with standard 8 ft walls. The perimeter is 2 × (12 + 10) = 44 ft, and 44 × 8 = 352 sq ft of wall (about 32.7 m²). That figure assumes four flat walls of the same height. A vaulted ceiling, a bump-out or a deep closet needs its own measurements rather than being squeezed into the rectangle formula.
For an L-shaped or irregular room, treat each wall as its own rectangle: multiply its width by its height and add the results together. Count the side faces of an alcove or a column if you will paint them. A 2 ft deep alcove with 8 ft walls adds two faces of 2 × 8 = 16 sq ft each, or 32 sq ft in total. Where a ceiling slopes, measure the wall at its average height, or split it into a rectangle and a triangle (half of base × height).
Convert inches to decimal feet before you multiply: 6 in is 0.5 ft, 9 in is 0.75 ft, so a wall 8 ft 6 in tall is 8.5 ft. The nearest inch is precise enough.
| Measurement | How to take it | What it is used for |
|---|---|---|
| Room length | Along the floor, corner to corner | Perimeter |
| Room width | Along the floor, across the other direction | Perimeter |
| Wall height | Floor to ceiling | Wall area (perimeter × height) |
| Doors and windows | Count them, or measure width × height | Area to subtract |
| Alcoves and bump-outs | Width × height of each extra face | Area to add |
Account for doors and windows
The gross figure includes the space taken up by doors and windows, which you will not paint. The quickest approach is a standard allowance per opening. Published allowances differ a little: Behr's interior guide uses about 20 sq ft for an average door and 15 sq ft for a window, This Old House suggests 20 sq ft per door and 10 sq ft per window, and the calculator on this site uses 20 sq ft per door and 12 sq ft per window. For one or two openings the difference is a few square feet, well under a quart of paint. Benjamin Moore's paint calculator also works from a simple count of doors and windows, and it assumes two coats and leaves the ceiling out.
If a room has a patio door, a bay window or a wide opening, measure it instead. A 3 × 7 ft door is 21 sq ft; a 6 × 6.7 ft sliding door is about 40 sq ft, roughly two standard doors. Enter a large opening as the number of standard doors or windows that comes closest, or subtract the measured area yourself and use the calculator's wall-area mode.
Do not subtract an opening twice. If you measured only the wall sections around a window, its area is already gone. The trim around doors and windows is a separate job, usually done in a different sheen, so leave it out of the wall estimate and buy trim paint by the quart. The calculator stops with an error if doors and windows add up to the whole wall area or more, and warns when they take an unusually large share, because that usually means a count was typed wrong.
| Source | Door | Window |
|---|---|---|
| This calculator | 20 sq ft | 12 sq ft |
| Behr interior paint guide | 20 sq ft | 15 sq ft |
| This Old House | 20 sq ft | 10 sq ft |
| Measured 3 × 7 ft door or 3 × 5 ft window | 21 sq ft | 15 sq ft |
Turn square feet into gallons
With the paintable area in hand, the formula is paintable area × coats ÷ coverage per gallon. Use the coverage printed on the can or the product's data sheet, not a general figure, because it varies by product line: Sherwin-Williams gives about 350 to 400 sq ft per gallon as typical, and Behr quotes 250 to 400 sq ft for its interior lines. Most walls get two coats; the guide on how many coats of paint you need covers when one or three makes more sense.
The surface matters too. New drywall soaks up more of the first coat, and textured walls have more surface than the tape measure shows. The calculator lowers the coverage to 80% for new drywall or plaster and to 75% for a textured or rough wall. How paint coverage works explains where those adjustments come from.
The calculator rounds the result to a tenth of a gallon and then plans cans: 5-gallon buckets once the job reaches 5 gallons, then whole gallons, then quarts for a small remainder (one quart is a quarter of a gallon). A remainder up to 0.25 gal becomes 1 quart, up to 0.5 gal becomes 2 quarts, and anything over half a gallon becomes another gallon. Switch on the Add 10% extra option if you want a margin for spills, roller soak-up and touch-ups. This Old House suggests keeping one or two extra quarts for later repairs.
Worked example
Back to the 12 × 10 ft bedroom with 8 ft walls: 2 × (12 + 10) × 8 = 352 sq ft of wall. It has one door and two windows. Using the calculator's allowances, that is 20 + 2 × 12 = 44 sq ft of openings, leaving 352 − 44 = 308 sq ft to paint.
The walls are smooth and already painted in a similar color, so the surface factor is 1. For two coats of a paint rated at 400 sq ft per gallon: 308 × 2 ÷ 400 = 1.54 gallons, which rounds to 1.5 gal. That is 1 gallon plus a 0.5 gal remainder, so the can plan is 1 gallon and 2 quarts. With the 10% extra option on, 1.54 × 1.1 = 1.69, which rounds to 1.7 gal; the 0.7 remainder is over half a gallon, so the plan becomes 2 gallons.
If the same room had new, unpainted drywall, the effective coverage would be 400 × 0.8 = 320 sq ft per gallon, and 308 × 2 ÷ 320 = 1.925, or 1.9 gal: the remainder is over half a gallon, so you would buy 2 gallons, plus primer as the product directs. Put your own measurements into the paint calculator to see the working step by step, then price the result with the paint cost calculator.