How to use the paint coverage calculator
- Choose imperial (ft, gallons) or metric (m, litres).
- Add each wall you will paint with its length and height.
- Enter the number and size of doors and windows to leave unpainted.
- Enter coats, the coverage per unit printed on your paint label, and a waste allowance.
- List the container sizes your store sells; the tool finds the combination that covers what you need with the least paint left over.
Worked example
Four walls (12, 12, 10, 10 ft) × 8 ft, one door, two windows, two coats
Gross wall area 352 sq ft. Minus one 3 × 7 ft door (21 sq ft) and two 3 × 5 ft windows (30 sq ft) = 301 sq ft. Two coats at 350 sq ft per gallon = 1.72 gal; plus 10% waste = 1.89 gallons. With 0.25, 1 and 5 gallon containers the least-volume purchase is 2 × 1 gallon. The door and window sizes are example values, and 350 sq ft per gallon is a label figure to check against your product.
How it works
Wall area = Σ length × height. Openings = doors × door area + windows × window area. Net area = max(0, wall area − openings). Paint = net area × coats ÷ coverage × (1 + waste). Ceilings are calculated the same way with their own coats and coverage. Containers: the combination of your sizes with the smallest total volume that covers the need, then the fewest containers. 1 US gal = 3.78541 L, 1 m² = 10.7639 sq ft.
Assumptions
- Coverage per coat comes from your paint label. Many labels state roughly 350 to 400 sq ft per US gallon (about 9 to 10 m² per litre); treat this as a convention to check, not a fact about your product.
- Door and window sizes are example values; measure your own.
- Waste percentage is your own allowance.
- Container sizes are whatever you enter from your store.
Frequently asked questions
Why does my paint label give a range?
Coverage depends on surface texture, porosity, application method and colour change. Use the lower end for rough or porous surfaces, or when covering a strong colour.
Do I need primer?
Often for new plaster or drywall, stains, glossy surfaces or large colour changes. Primer has its own coverage; run the calculator again with the primer label figure.
Why is the ceiling separate?
Ceilings are often painted with a different product and number of coats, so the tool reports them separately and rounds each to containers.
Limitations
- Does not estimate trim, doors or cabinetry paint.
- Assumes a flat surface; heavy textures use more paint.