Work out litres or gallons of paint for your rooms, minus doors and windows, with coats, ceilings and can sizes.
The paint you need is the area to paint, times the number of coats, divided by what one litre or US gallon covers. For one room the wall area is 2 × (length + width) × height. A room of 4 m by 3.5 m with 2.5 m walls has 2 × 7.5 × 2.5 = 37.5 m² of wall. One door of 1.6 m² and one window of 1.5 m² are taken off, leaving 34.4 m². Two coats make 68.8 m², and at 10 m² per litre that is 6.88 litres. The page then rounds up to whole cans: 7 cans of 1 litre, 3 of 2.5 litres, 2 of 5 litres or 1 of 10 litres. In feet the same idea applies: 13 ft by 12 ft with 8 ft walls is 400 sq ft, minus a 20 sq ft door and a 15 sq ft window is 365, two coats is 730, and at 350 sq ft per gallon that is 2.09 US gallons. The box under the answer shows each step.
Coverage is a box you can edit. The metric default is 10 m² per litre per coat, which is about 407 sq ft per US gallon. The imperial default is 350 sq ft per US gallon. Sherwin-Williams states that a gallon of paint typically covers about 350 to 400 square feet, and says the top figure is theoretical and does not allow for surface texture or paint left on tools. Rough or porous surfaces use more, so use the coverage printed on your can when you have it. Coats multiply the area, from 1 to 5, with 2 as the default. Turn on the ceilings option and each room's length × width is added once: in the metric example that adds 14 m² and the answer becomes 9.68 litres. The ceiling is taken as the same paint as the walls, so work it out separately if you use a different one.
Up to 4 rooms can be added, each with its own length, width and height. A room with some of the three sizes filled in shows a message under the missing ones, and a room with none is skipped. Doors and windows are taken off the wall total as a count times an area, and both areas are editable because sizes vary. The defaults are 1.6 m² and 1.5 m², or 20 sq ft and 15 sq ft, which are example sizes rather than standards. If the doors and windows add up to more than the walls, the page says so instead of showing a negative amount. Skirting boards, trim, radiators, a sloped ceiling and the paint a roller soaks up are not counted.
Cans are shown for each common size by itself, so you can compare, not as a mixed basket. The counts come from dividing the paint needed by the can size and rounding up, with at least one can when any paint is needed. The metric sizes are 1, 2.5, 5 and 10 litres, and the imperial ones are a quart, a US gallon and a 5 US gallon pail. Gallons here are US gallons (3.785 L); a UK imperial gallon is 4.546 L, so 6.88 L is 1.82 US gallons but 1.51 UK gallons. If the paint needed is zero, for example when doors and windows cover all the wall, no cans are shown as needed; otherwise at least one can is. Check which sizes your shop sells, because they differ by brand and country. The result is an estimate. Many people buy a little more than the figure for touch-ups, and the exact amount is for the paint's label to decide.
With 2.5 m walls, one door of 1.6 m², one window of 1.5 m² and two coats at 10 m² per litre, you need 6.88 litres for the walls, or 9.68 litres if you also paint the 14 m² ceiling. That is 7 cans of 1 litre or 3 cans of 2.5 litres for the walls.
With 8 ft walls, a 20 sq ft door, a 15 sq ft window, two coats and 350 sq ft per gallon, you need 2.09 US gallons. That is 3 one-gallon cans. At 400 sq ft per gallon the same room needs 1.83 US gallons. The page uses US gallons, not UK imperial gallons.
Add the length and width, double it, and multiply by the height: 2 × (length + width) × height. For 4 m by 3.5 m and 2.5 m high that is 2 × 7.5 × 2.5 = 37.5 m². Subtract doors and windows to get the area to paint.
The page starts at 10 m² per litre per coat, and 350 sq ft per US gallon in feet. Sherwin-Williams gives about 350 to 400 sq ft per gallon. Your can's figure is the one to type in, since it depends on the paint and the surface.
The page starts with 2, which doubles the area: 34.4 m² of wall becomes 68.8 m² of coverage. Change Coats from 1 to 5. A dark colour over a light one or bare plaster may need more than a repaint in the same colour.
No. The calculation runs in your browser and the sizes stay in this tab. They are not uploaded or stored on your device. Reloading the page puts the example back. The page only counts that a result was shown, without any of your numbers.