How paint quantity is calculated
A paint calculator works out the wall area you'll actually paint, multiplies it by the number of coats, and divides by the paint's coverage rate, the area one liter covers in a single coat. This one subtracts doors and windows first and gives the answer in liters and US gallons, so you can match it to tin sizes.
The formula
- Gross wall area = Wall length × Wall height × Number of walls
- Openings = (Doors × Area per door) + (Windows × Area per window)
- Paintable area = Gross wall area − Openings (never below zero)
- Paint required (L) = Paintable area × Coats ÷ Coverage rate
- Paint required (US gal) = Liters ÷ 3.785411784
The defaults are 4 walls, 2 coats, 10 m² per liter, 2.0 m² per door and 1.5 m² per window. All of them are editable. Results are rounded to two decimal places.
The calculator multiplies one wall length by the number of walls, so it assumes the walls are the same size. For a rectangular room with two different wall lengths, enter the room's full perimeter as the wall length and set the number of walls to 1.
Worked example
A 5 m × 4 m bedroom with 2.7 m ceilings, one door and one window, two coats, and paint that covers 10 m² per liter:
- Perimeter: 5 + 4 + 5 + 4 = 18 m. Enter 18 m as the wall length and 1 wall.
- Gross area: 18 × 2.7 = 48.6 m².
- Openings: 1 × 2.0 + 1 × 1.5 = 3.5 m².
- Paintable area: 48.6 − 3.5 = 45.10 m².
- Paint: 45.1 × 2 ÷ 10 = 9.02 L, or 2.38 US gallons.
In practice you'd buy two 5-liter tins, or a 10-liter tin, and keep the leftover for touch-ups.
Coverage rates from the label
Coverage varies with the product, the finish and the surface, so the most reliable number is the one on the tin. Sherwin-Williams, for example, tells customers to check the label for the square feet a gallon covers. If your label uses US units, this table converts it to the m²/L the calculator expects:
| Label coverage | Enter as coverage rate |
|---|---|
| 250 ft² per gallon | 6.1 m²/L |
| 300 ft² per gallon | 7.4 m²/L |
| 350 ft² per gallon | 8.6 m²/L |
| 400 ft² per gallon | 9.8 m²/L |
The conversion is 1 ft²/gal = 0.0929 m² ÷ 3.785 L ≈ 0.0245 m²/L.
When to adjust the inputs
- New plaster or bare drywall soaks up paint. Use a primer or mist coat first, and expect lower coverage from it than the label rate for finished walls.
- Strong colour changes, such as light over dark, often need a third coat. Set coats to 3.
- Textured or rough surfaces have more surface area than their flat dimensions suggest, so lower the coverage rate.
- Spraying typically uses more paint than a roller because of overspray.
How to measure
- Measure the length of each wall at floor level and add them up for the perimeter.
- Measure the height from the floor to the ceiling (or to the top of the area you're painting).
- Count doors and windows. If yours are much larger or smaller than 2.0 m² and 1.5 m², measure one and enter its width × height.
- Read the coverage rate from your chosen paint's label or data sheet.
Related calculators
- Room calculator: floor, ceiling and wall areas for the whole room, including the ceiling you'll paint separately.
- Plaster calculator: if the walls need plastering before you paint.
- Volume converter: convert liters to gallons, or between other volume units.
- Area converter: switch between m² and ft².
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