Wall & Paint Sqft Calculator
Enter the room perimeter and wall height, then the number of doors and windows to subtract. Paint quantity is worked out automatically.
To calculate wall square footage, multiply the room perimeter by the wall height, then subtract doors and windows. A room with a 52 ft perimeter and 8 ft ceilings has 416 sq ft of wall; subtracting one door (21 sq ft) and two windows (30 sq ft) leaves 365 square feet to paint.
Allowances used: 21 sq ft per door, 15 sq ft per window, 350 sq ft coverage per gallon per coat.
How to calculate wall square footage
- Measure the perimeter. Add the lengths of all the walls. For a 14 ft × 12 ft room that is 14 + 12 + 14 + 12 = 52 ft.
- Multiply by the wall height. 52 × 8 = 416 sq ft of gross wall area.
- Subtract the openings. A standard door is about 21 sq ft (3 ft × 7 ft) and an average window about 15 sq ft.
For a single wall rather than a whole room, it is simply width × height.
When to subtract doors and windows
It depends on how much you are painting and how precise you need to be.
- One or two small windows — many decorators skip the subtraction entirely, treating it as built-in waste allowance.
- Large windows, patio doors, or several openings — always subtract. A patio door alone can be 40 sq ft, which is a meaningful share of a gallon.
- Tiling or drywall — always subtract, since material cost per square foot is much higher than paint.
How much paint you need
One US gallon covers roughly 350 square feet per coat on a smooth, previously painted surface. Plan for two coats in almost every case.
So 365 sq ft of wall over two coats is 730 sq ft of coverage, which is 730 ÷ 350 = 2.09 gallons — buy 3 gallons, or 2 gallons plus a quart.
Coverage drops on textured walls, bare drywall or when making a dramatic colour change. Deep reds and bright yellows in particular often need three coats over a light base.
Wall area by room size
Gross wall area with 8 ft ceilings, before subtracting openings.
| Room size | Perimeter | Wall area | Gallons (2 coats) |
|---|---|---|---|
| 10 × 10 ft | 40 ft | 320 sq ft | 1.8 gal |
| 12 × 12 ft | 48 ft | 384 sq ft | 2.2 gal |
| 12 × 14 ft | 52 ft | 416 sq ft | 2.4 gal |
| 14 × 16 ft | 60 ft | 480 sq ft | 2.7 gal |
| 16 × 20 ft | 72 ft | 576 sq ft | 3.3 gal |
| 20 × 24 ft | 88 ft | 704 sq ft | 4.0 gal |
Frequently asked questions
How do I calculate the sqft of a wall?
Multiply the wall width by its height. For a whole room, add all the wall lengths to get the perimeter, multiply by the ceiling height, then subtract doors and windows.
How much wall area does a gallon of paint cover?
About 350 square feet per coat on a smooth, previously painted surface. Bare drywall, textured walls and dramatic colour changes all reduce coverage.
Should I subtract windows and doors?
For one or two small windows many decorators do not bother, treating it as a waste allowance. Always subtract large openings such as patio doors, and always subtract when calculating tile or drywall.
How many square feet is a standard door?
A standard interior door is about 21 square feet — 3 feet wide by 7 feet tall. An average window is around 15 square feet.
How do I calculate paint for the ceiling too?
The ceiling is the room floor area — length × width. Add that to the wall total before dividing by the coverage rate.