How to use
- Enter the room’s length, width and uniform wall height. Select the ceiling only if it will use the same paint product.
- Add doors and windows, including the number of identical openings. Remove rows that do not apply.
- Enter the number of coats and the coverage for one coat, using your product label. Add an allowance suited to your project.
- Enter one container size and its price. The purchase estimate rounds up to whole containers of that size.
- Check the paintable area, required volume and purchased volume. Save, copy or print the result.
Metric uses metres, square metres and litres. US uses feet, square feet and US gallons. A US gallon is 3.785411784 litres; this calculator does not use Imperial gallons. Currency and units are independent, and no currency conversion takes place.
Calculation method
- Gross wall area = 2 × (room length + room width) × wall height.
- Opening area = sum of opening width × height × quantity.
- Paintable area = gross wall area − openings + optional ceiling area.
- Base paint volume = paintable area × coats ÷ coverage per coat.
- Required volume = base volume × (1 + allowance ÷ 100).
- Containers to buy = required volume ÷ container size, rounded up.
The ceiling is room length × room width. Openings are subtracted from the walls, not from the ceiling. If the openings exceed the gross wall area, the form asks you to correct the inputs rather than returning a misleading estimate.
A worked example
Take a room 5 × 4 × 2.6 m, with no ceiling included. The walls total 46.8 m². One 0.9 × 2.1 m door and two 1.2 × 1.2 m windows deduct 4.77 m², leaving 42.03 m² to paint.
At two coats and 10 m² per litre per coat, the base volume is 8.406 L. Adding 10% gives 9.2466 L. With 5 L containers, buy 2 containers, or 10 L. At 45 USD each, the paint costs 90 USD.
The on-screen volume is rounded to 9.25 L, while the purchase calculation uses the unrounded value. Coverage and price here are example inputs, not claims about a particular product.
Understanding your result
The required paint volume includes your chosen allowance. The purchase quantity adds any extra volume caused by rounding to whole containers. That difference is an estimate of excess purchased volume, not a promise about paint left after application.
One calculation models one product and one container size. If the ceiling uses a different paint, calculate it separately. The tool does not combine different tin sizes to minimise the price. Primer, trim, repairs, labour, tax and delivery are excluded.
Real coverage depends on the coating, surface and application. Read the label and consult the paint coverage and coats guide before deciding what to order.