Calculator — Paint & Walls
Wall Area Calculator
Net wall area for any room — perimeter times ceiling height minus doors and windows — the feeder figure for paint, primer, drywall and wallpaper.
Inputs
Results
Adjust the inputs to see your result.
About this calculator
Every wall job starts from one number: the net wall area. This calculator finds it from room size and ceiling height, deducts standard doors and windows, and reports the ceiling area separately so paint, primer, drywall and wallpaper tools each get the figure they actually need.
What this number means
Almost every wall job starts with the same number — the net wall area — and almost every bad estimate starts from skipping it. Paint, primer, drywall and wallpaper calculators all ask for it, and guessing it is how a two-gallon job becomes a four-gallon job or a Saturday job becomes a Sunday store run.
The two subtleties are the deductions and the ceiling. Doors and windows are not painted, so they come off: a standard door is 21 sq ft, a standard window 15. And the ceiling is a separate surface — 180 sq ft in the worked example — which matters for paint and primer but not for drywall sheets counted by wall area alone. This calculator reports all three figures so each downstream tool gets the number it actually needs.
The formula
- Perimeter = 2 × (Length + Width) (ft)
- Gross wall area = Perimeter × Ceiling height (sq ft)
- Openings = Doors × 21 + Windows × 15 (sq ft)
- Net wall area = Gross wall area − Openings
- Ceiling area = Length × Width (sq ft)
- Total paintable area = Net wall area + Ceiling area
Worked example
A 12 × 15 ft room, 8 ft ceilings, 1 door, 2 windows.
- Perimeter: 2 × (12 + 15) = 54 ft
- Gross wall area: 54 × 8 = 432 sq ft
- Openings: 1 × 21 + 2 × 15 = 51 sq ft
- Net wall area: 432 − 51 = 381 sq ft
- Ceiling area: 12 × 15 = 180 sq ft
- Total paintable area: 381 + 180 = 561 sq ft
Assumptions and limitations
- The room is rectangular. For an L-shaped or irregular room, split it into rectangles, run each one through this calculator, and add the net wall areas — or measure the total wall length and use it in place of the perimeter.
- Door and window deductions use standard sizes (3 × 7 ft door, 3 × 5 ft window). Measure oversized openings — French doors, picture windows, archways — and adjust the counts or the deduction by hand.
- Sloped or vaulted ceilings are not modelled: use the average wall height for a rough figure, or measure each wall separately for an exact one.
- Baseboards, trim and wainscoting are not deducted — they are small against the wall total and are usually painted with the wall anyway.
- Closets count as rooms of their own; their walls are not included in the main room unless you measure them in.
Frequently asked questions
Multiply the room perimeter by the ceiling height, then subtract 21 sq ft per door and 15 sq ft per window. A 12 × 15 ft room with 8 ft ceilings, 1 door and 2 windows: 432 − 51 = 381 sq ft of net wall area.
Yes for paint, primer and wallpaper — you do not paint a door opening. A standard door deducts 21 sq ft and a standard window 15 sq ft. For drywall, some installers skip the deduction because sheets are cut around openings anyway.
No — wall area and ceiling area are reported separately here. Add them together (561 sq ft in the worked example) when the ceiling is being painted or primed too; use the wall figure alone for wallpaper and wainscoting.
Split the room into rectangles and add the results, or walk the walls with a tape and add up every wall length — that total replaces the perimeter in the formula. The deduction step (doors and windows) stays the same.
The net wall area from this calculator — 381 sq ft in the worked example. The Paint Calculator then applies coats and coverage per gallon to that figure, and the Primer Calculator does the same for primer.
About 350 sq ft per coat on smooth walls, less on textured or unpainted surfaces. So the worked example's 381 sq ft needs just over one gallon per coat — two coats means 3 gallons after rounding up.