Wall & Room Square Footage Calculator
Square footage is the foundation of every paint and cost estimate — get it right first and everything downstream falls into place. Measure the perimeter and the wall height; this returns the gross wall area before you subtract doors and windows.
A 12 ft × 15 ft room with 8 ft walls is about 432 square feet of wall (perimeter × height), before you subtract doors and windows. Square footage is the base of every paint and cost estimate — measure the perimeter and the wall height.
Calculator inputs
Before you can size paint, primer or a painter’s quote, you need the wall area. For a rectangular room it is the perimeter (all four walls, laid end to end) multiplied by the wall height. This is the gross figure — the paintable area comes after you deduct the doors and windows, which the how-much-paint calculator and room paint calculator do for you.
Why perimeter × height and not floor area? Because you paint the walls, not the floor. A long, narrow room and a square room with the same floor area have very different wall areas — the perimeter is what matters.
Formula
perimeter = 2 × (length + width)gross wall area = perimeter × wall height
For an L-shaped or irregular room, add up each wall’s length by hand, then multiply the total run by the height.
Worked example
A 12 ft × 15 ft room with 8 ft ceilings:
- Perimeter = 2 × (12 + 15) = 54 ft.
- Gross wall area = 54 × 8 = 432 sq ft.
That 432 sq ft is what you carry into the paint and cost tools — then knock off ~21 sq ft per door and ~15 per window for the paintable area.
Measure the perimeter first
Measuring cleanly here saves every downstream number:
- Measure the perimeter, not the floor diagonal. Walk the tape along the base of each wall.
- Handle odd shapes wall by wall. Add each wall length to get the total run, then multiply once by the height — do not average.
- Vaulted or sloped ceiling? Split the gable triangle out (½ × base × height) and add it to the rectangular part.
- Keep gross and net separate. Square footage here is gross; deduct openings only when you size the paint.
Reference table
| Opening | Deduct (sq ft) |
|---|---|
| Standard interior door (3 × 7) | 21 |
| Standard window | 15 |
| Patio / sliding-glass door | 40 |
Standard deductions — measure your actual openings and adjust. See door & window deductions.
Frequently asked questions
What is the square footage of a 12x15 room?
With 8 ft walls the wall area is 2 × (12 + 15) × 8 = 432 sq ft gross. The floor is 12 × 15 = 180 sq ft, which is also the ceiling area — but for wall paint you want the 432 figure.
Do I subtract doors and windows here?
Not on this tool — it returns gross wall area on purpose, so you can reuse it for walls, ceilings or a quote check. Subtract ~21 sq ft per door and ~15 per window when you size paint with the how-much-paint calculator.
How do I measure an irregular or L-shaped room?
Measure each straight wall segment, add the lengths for the total run, and multiply that run by the wall height. For a gable or vaulted section, add the triangle area (½ × base × peak height) on top of the rectangular walls.