Wall Paint Calculator
Calculate paint needed for walls, accounting for doors and windows.
Last updated 2026-08-21
What Is a Wall Paint Calculator?
The wall paint calculator figures out how much paint covers just the walls of a room, subtracting doors and windows so you do not over-order. You enter the room length, wall height, coats, and the square footage of your doors and windows. It is the fastest way to size paint for a single room paint job.
How It Works
Wall area = 4 × Length × Height. Paintable = Wall area − Doors − Windows. Gallons = ceil((Paintable × Coats) ÷ 350).
Example
A 20 ft × 8 ft room: 640 sq ft of wall, minus 36 sq ft of doors/windows = 604 sq ft → 4 gallons for 2 coats.
How to Calculate
- Enter the room length. For a 20 ft by 8 ft room, the calculator multiplies by 4 to assume four walls, giving 4 × 20 × 8 = 640 sq ft of wall.
- Add the area of your doors and windows. A standard door is about 21 sq ft and a typical window 12 to 15 sq ft. If your room has one door and one 15 sq ft window, subtract 36 sq ft.
- Subtract to get the paintable area: 640 − 36 = 604 sq ft.
- Set the Number of Coats, say 2. Multiply the paintable area by coats: 604 × 2 = 1,208 sq ft of coverage needed.
- Divide by the coverage rate of 350 sq ft per gallon: 1,208 ÷ 350 = 3.45, so round up to 4 gallons.
Typical door and window areas
| Opening | Typical area |
|---|---|
| Standard interior door | 21 sq ft |
| Double or French door | 40 sq ft |
| Small window | 12 sq ft |
| Standard window | 15 sq ft |
| Large picture window | 24 to 30 sq ft |
Sources
Frequently Asked Questions
Why multiply by 4?
This assumes a rectangular room with four equal walls of the entered length. For irregular rooms, measure each wall.
Should I subtract doors and windows?
Yes, subtract their area to avoid over-ordering. The tool accepts both as inputs.
What is a typical door/window area?
A standard door is about 21 sq ft; a typical window 12–15 sq ft.
Estimate the cost with the Paint Cost Calculator or browse Paint Calculators