Transparent calculation
Wall-paint formula
The calculation starts with the perimeter of a rectangular room, subtracts rule-of-thumb unpainted openings, then applies coats, product coverage, and the visible waste allowance.
Gross wall area = 2 × (length + width) × height
Opening allowance = doors × 20 sq ft + windows × 15 sq ft
Paintable area = gross wall area − opening allowance
Gallons before waste = paintable area × coats ÷ coverage
Gallons with waste = gallons × (1 + waste % ÷ 100)
Whole gallons to buy = round gallons with waste up
Worked example
For a 12 ft × 10 ft room with 8 ft walls, one door, two windows, two coats, 400 sq ft/US gal coverage, and 10% waste:
- Paintable walls
- 302 sq ft
- Calculated quantity
- 1.66 US gal · 6.29 L
- Whole-gallon suggestion
- 2 US gal
Paint quantity questions
Those are documented rule-of-thumb opening areas used for a quick room estimate. Measure unusually large or small openings separately when you need a more precise material plan.