Gallons or litres: measure the walls, subtract doors and windows, apply the coverage rate and multiply by the coats.
Estimating paint is easier than it looks. Whether you work in feet, square feet and gallons or metres, square metres and litres, the method is the same: measure the wall area, subtract doors and windows, apply the paint's coverage rate, then multiply by the number of coats.
Below is the full process in both systems, starting with the Imperial (US) method and then the metric (UK/EU) method, each with formulas, coverage rates and a worked example.
You need the total wall area in square feet.
Wall area (ft²) = (perimeter × height) − openings
Coverage varies by brand and quality. These are the usual US ranges:
| Paint type | Coverage per gallon |
|---|---|
| Standard interior paint | 350–400 ft² |
| Premium paint | 400–450 ft² |
| Budget paint | 250–300 ft² |
Surface texture matters too. Rough walls absorb more paint than smooth ones.
Most rooms need two coats for an even finish. Plan on three when you are:
Gallons needed = wall area (ft²) ÷ coverage (ft² per gallon) × coats
The room is 13 ft × 11.5 ft with an 8 ft ceiling.
| Step | Working | Result |
|---|---|---|
| Perimeter | 13 + 11.5 + 13 + 11.5 | 49 ft |
| Wall area | 49 × 8 | 392 ft² |
| Minus door and window (about 35 ft²) | 392 − 35 | 357 ft² |
| Coverage | Standard paint | 350 ft² per gallon |
| Gallons, two coats | 357 ÷ 350 × 2 | 2.04 gallons |
Result: about 2 gallons. A typical purchase is two 1-gallon cans. Two gallons is 7.57 litres, and you can check other amounts with the gallons to litres converter.
Ceiling area is length × width, and coverage is usually 350 to 400 ft² per gallon. Ceilings often take slightly more paint because the matte finish is absorbent.
| Room size | Paint needed |
|---|---|
| Small room | 1–1.25 gallons |
| Medium room | 1.5–2 gallons |
| Large room | 2.5–3 gallons |
In the metric system the wall area is in square metres (m²).
Wall area (m²) = (perimeter × height) − openings
These are the US coverage rates converted to metric, so both systems describe the same paint. One US gallon is 3.785 litres and one square foot is 0.0929 m², which makes 350 ft² per gallon about 8.6 m² per litre.
| Paint type | Coverage per litre | Same rate in US units |
|---|---|---|
| Standard emulsion | 8.6–9.8 m² | 350–400 ft² per gallon |
| Premium paint | 9.8–11.0 m² | 400–450 ft² per gallon |
| Budget paint | 6.1–7.4 m² | 250–300 ft² per gallon |
As with US paint, coverage depends on the surface texture and how thick the paint is.
Two coats are standard for most walls. Use three when you are:
Litres needed = wall area (m²) ÷ coverage (m² per litre) × coats
The room is 4 m × 3.5 m with a 2.4 m ceiling.
| Step | Working | Result |
|---|---|---|
| Perimeter | 4 + 3.5 + 4 + 3.5 | 15 m |
| Wall area | 15 × 2.4 | 36 m² |
| Minus door and window (about 3.4 m²) | 36 − 3.4 | 32.6 m² |
| Coverage | Standard emulsion | 8.6 m² per litre |
| Litres, two coats | 32.6 ÷ 8.6 × 2 | 7.6 litres |
Result: about 7.6 litres. A typical purchase is a 5 L, a 2.5 L and a 1 L tin, or a single 10 L tin. 7.6 litres is 2.0 US gallons, in line with the US example, and the litres to gallons converter handles other amounts.
Ceiling area is length × width, with coverage of about 8.6 to 9.8 m² per litre (the same 350 to 400 ft² per gallon range). Ceilings often take slightly more paint because of how absorbent they are.
| Room size | Paint needed |
|---|---|
| Small room | 3.8–4.7 litres |
| Medium room | 5.7–7.6 litres |
| Large room | 9.5–11.4 litres |
Gallons or litres, the calculation is the same:
Most rooms fall between 1 and 2 gallons, or about 4 to 8 litres, for two coats of interior paint. Larger rooms or low-coverage paints may need more.