Start with the right measurement
Paint quantity is driven by paintable wall area, not floor area. Start with the perimeter, subtract doors and windows, then let the product label’s coverage rate control the final container estimate.
How to use this calculator
- 01Perimeter and height: Total wall length and finished height for the room or zone.
- 02Openings: Combined door and window area to subtract.
- 03Coats, coverage, container: Use the exact product label and desired finish.
The calculated coverage is shown before rounding to whole containers. Coverage rates vary with texture, porosity, colour, application method, and the manufacturer’s product.
Units: Imperial dimensions use feet and square feet; metric dimensions use metres and square metres. Convert openings to the same area unit before subtracting.
How the calculation works
Paint = (wall area − openings) × coats ÷ coverage rate. Containers = ceiling(paint needed ÷ container size).A 40 ft perimeter at 8 ft high, minus 40 ft² of openings, needs 560 ft² of coverage for two coats.
Worked examples
One bedroom
- Measure a 42 ft perimeter and 8 ft height: 336 ft² of wall area.
- Subtract a 21 ft² door and two 15 ft² windows: 285 ft² paintable.
- At 350 ft² per gallon and two coats, divide 570 by 350.
Result: Plan on 2 gallons, then confirm the product’s coverage and colour change guidance.
Several walls with a large opening
- Use a 30 ft perimeter and 9 ft height: 270 ft².
- Subtract a 60 ft² opening: 210 ft².
- At 400 ft² per gallon for one coat, 210 ÷ 400 rounds up to one gallon.
Result: A second coat doubles the paintable coverage requirement; the calculator keeps coats explicit.
How to measure
- 01Measure total wall perimeter and finished wall height.
- 02Add door and window areas to subtract them.
- 03Use the manufacturer’s stated coverage for the exact product and surface.
Common mistakes to avoid
- Forgetting the second coat
- Using floor area as wall area
- Assuming label coverage applies to every surface
Safety note
For older homes, follow local guidance about lead paint before sanding or disturbing existing coatings; ventilate and use the product safety information.
Limits of this estimate
Texture, porosity, colour change, application method, and waste affect coverage. Verify the product label.
Frequently asked questions
Should I subtract windows?
For a closer estimate, subtract their combined area. Many painters leave small openings in as a practical allowance.
What coverage should I use?
Use the manufacturer’s stated rate for the exact product and surface.