Wall Paint Calculator
Wall paint = (perimeter × wall height − about 21 ft² per door and 15 ft² per window) × coats ÷ spread rate. A 12 × 16 ft room with 8 ft walls, one door and two windows has 397 ft² to paint: 2.27 gallons for two coats at 350 ft²/gal. For a measured area, see the Paint Calculator.
How it's calculated: Gallons = (perimeter × height − 21 ft² per door − 15 ft² per window) × coats ÷ spread rate.
Result appears here
Fill in the fields and press Calculate.
Formula, calculation, assumptions and sources
Gross wall area = Perimeter × Height
Paintable area = Gross − (doors × 21 ft²) − (windows × 15 ft²)
Gallons = (Paintable area × coats) ÷ spread rate
Run the calculation to see each step with your own numbers.
A 12 ft × 16 ft room, 8 ft walls (56 ft perimeter), 1 door, 2 windows, 2 coats.
Gross = 56 × 8 = 448 ft²
Less 21 + 30 = 51 ft² → 397 ft²
Gallons = (397 × 2) ÷ 350 = 2.27 → 3 gal (or 2 gal + 2 qt)
- Doors estimated at 21 ft² and windows at 15 ft² each — adjust with the Wall Square Footage Calculator if yours differ a lot.
- Ceiling and trim are separate.
- Assumes a primed or previously painted surface (spread rate 350 ft²/gal).
- It came out to 2.27 gallons — round to 3?
- 2 gallons + 2 quarts is the economical buy (a quart ≈ 90 ft²). Round to 3 whole gallons only if quarts aren't available or you want extra.
- How big are the deductions for doors and windows?
- About 21 ft² per door and 15 ft² per window — standard sizes. If your openings are unusually large, work out the real wall area with the Wall Square Footage Calculator instead.
- How much paint do I need for a 12x16 room?
- About 2.27 gallons for two coats on 8 ft walls with one door and two windows: 448 ft² of wall less 51 ft² of openings leaves 397 ft². Buy 2 gallons and 2 quarts, or 3 gallons.
Spread rate
≈ 350 ft² per US gallon per coat for interior latex on a primed, smooth surface. Paint-manufacturer data sheets (Sherwin-Williams, Benjamin Moore) quote 350–400. Rougher/porous surfaces are lower. Editable.
Opening sizes
Standard interior door ≈ 3 × 6.7 ft ≈ 21 ft²; typical window ≈ 15 ft² (approximations for estimating).
About the Wall Paint Calculator
The Wall Paint Calculator sizes paint for the walls of a room from the room's perimeter and height and the number of doors and windows, deducting a standard area for each opening. It is the quick version of the Paint Calculator for when you have a room rather than a measured wall area. Ceiling and trim are separate.
What you enter
- Room perimeter — Add up all the wall lengths. A 12×12 room = 48 ft.
- Wall height
- Doors
- Windows
- Coats
- Coverage (ft² per gallon per coat) — Manufacturer's spread rate. 350–400 for smooth walls; 250–300 for textured, masonry or bare drywall.
What you get back
- Paintable area — the exact result of the calculation.
- Paint needed — Round up to whole cans — a gallon and a quart is often cheaper than two gallons.
The formula, a worked example, the assumptions behind it and the sources are shown in full below the calculator.
Related calculators
- Paint Calculator — Paint in gallons or litres for a project from surface area and coats.
- Ceiling Paint Calculator — Paint for a ceiling of a given area and number of coats.
- Wall Square Footage Calculator — Wall area for one or more walls, with openings deducted.
- Wallpaper Calculator — Wallpaper rolls for a room from wall area and pattern repeat.
Related articles
- How Much Paint Coverage Should You Expect? — Paint coverage is usually stated per gallon on the can, but surface texture, porosity and application method can all shift real-world coverage.
Part of the Painting calculators.