CRITERIA
1. Why subtract (without overdoing it)
A standard door is 20 sq ft of wall you won't roll. Two doors and three windows add up to a full wall's worth of paint. The subtraction sharpens the purchase — but the built-in 10% waste already cushions honest measuring errors.
SIZES
2. The reference sizes
Standard door: 3 × 6.67 ft = 20 sq ft. Standard window: 4 × 4 ft = 16 sq ft. Bigger units — patio doors, picture windows — should be measured for real; a tape measure costs less than the gallon you didn't need.
| Room | Typical openings | Deduct |
|---|---|---|
| Bedroom | 1 door + 1 window | −36 sq ft |
| Living room | 1 door + 2 windows | −52 sq ft |
| Kitchen | 1 door + 1 window | −36 sq ft |
The calculator's dimensions mode subtracts 20 sq ft per door and 16 sq ft per window automatically.
EXCEPTIONS
3. When not to subtract
If you will brush the frames in the same wall color, don't deduct: the brush uses what the roller saves. With many small openings the math isn't worth it — subtract half the opening area as a pragmatic middle ground.
EXAMPLE
4. A 12×12 room, start to finish
Walls: 4 walls × 12 ft = 48 ft perimeter × 8 ft height = 384 sq ft. One door (−20) and two windows (−32) leave 332 sq ft. Two coats at 350 sq ft/gal with 10% waste ≈ 1.9 gallons: buy 2.
Ready to skip the math?
Open the paint calculator