Paint Calculator

How it works

The room is treated as a box: four walls of the same height, and a ceiling of length × width. The paint you need is the area to cover, times the number of coats, divided by how much area one unit of paint covers in a single coat:

wall area = 2 × (length + width) × height − doors and windows
ceiling area = length × width
paint = area × coats / coverage

The calculator shows two quantities, the paint for the walls alone and for the walls plus the ceiling, since most rooms get both. Use the metric or imperial version: the units follow your region and can be switched at the top. Imperial results are in US gallons.

Only the three room measurements are required. The doors and windows, the number of coats and the coverage are optional: left empty they are 0, 2 coats, and a typical 10 m² per liter (350 ft² per US gallon). Check the coverage printed on your tin and enter it for a closer answer.

The result is the exact amount the arithmetic gives, not rounded up to tin sizes. Buying some extra is advice, not part of the calculation, and is covered below.

Coats, and primer as a separate job

The paint grows in direct proportion to the coats: three coats need exactly one and a half times as much as two. The calculator accepts 1 to 10 whole coats.

A primer is a different product with its own coverage figure, so do not fold it into the coat count of your topcoat. Run the calculation a second time with 1 coat and the coverage printed on the primer's label. The tool takes coverage between 1 and 30 m² per liter (the equivalent in square feet per gallon in the imperial version); a number outside that range is rejected as out of range, which usually means a unit slip.

Reading the coverage on the label

Labels state coverage in different ways: per liter, per tin, sometimes as a range, always for one coat on a stated kind of surface. Convert it to area per liter by dividing by the tin volume. A 5 liter tin said to cover 45 m² is 45 / 5 = 9 m² per liter. If the label gives a range, the lower end is the safer figure for rough or absorbent walls. The numbers in the examples here are for the arithmetic; your tin is the authority.

Walls and ceiling

The ceiling is always length × width and nothing is subtracted from it. The walls-and-ceiling figure assumes the same paint, coats and coverage for both. If the ceiling gets its own paint, take the ceiling figure from the difference: walls plus ceiling, minus walls alone.

Liters and US gallons

The imperial version works in feet, square feet and US gallons. One US gallon is exactly 3.785411784 liters and one square foot is exactly 0.09290304 m². The built-in defaults are not the same number in two systems: 350 ft² per gallon is 350 × 0.09290304 / 3.785411784, about 8.59 m² per liter, while the metric default is 10. A UK imperial gallon is a different, larger unit (4.54609 liters); if your tin is labeled in those, work in liters.

Examples

A bedroom. The room is 5 m long, 4 m wide and 2.5 m high. The walls are 2 × (5 + 4) × 2.5 = 45 m². A door and a window take out 3 m², which leaves 42 m². Two coats at 10 m² per liter need 42 × 2 / 10 = 8.4 liters. The 20 m² ceiling adds 4 liters for the same two coats, 12.4 liters in total.

A room in feet. A room of 16 × 12 ft with 8 ft walls has 448 ft² of wall. Taking out 40 ft² for openings leaves 408 ft². Two coats at 350 ft² per gallon need 408 × 2 / 350 = 2.33 gallons, or 3.43 gallons with the 192 ft² ceiling.

Buying the paint. The bedroom needs 8.4 liters for the walls. Add 10 percent and you want about 9.2 liters, which is four 2.5-liter tins (10 liters), or one 10-liter and one 2.5-liter tin if your supplier sells them.

Measuring the openings. Take the same bedroom with a door of 0.9 × 2.1 m (1.89 m²) and a window of 1.2 × 1.2 m (1.44 m²). Together they are 3.33 m², so the net wall area is 45 − 3.33 = 41.67 m². Two coats at 10 m² per liter need 41.67 × 2 / 10 = 8.334 liters, shown as 8.33. Ignoring the openings would give 90 / 10 = 9 liters, 0.67 liters more.

Primer first. The same 41.67 m² of wall with one coat of a primer whose label says 8 m² per liter (a made-up label, for the arithmetic): 41.67 / 8 = 5.21 liters of primer. Then run the topcoat separately with 2 coats.

Ceiling on its own. With walls at 8.334 liters and walls plus ceiling at 12.334 liters, the ceiling needs 12.334 − 8.334 = 4 liters.

Liters to gallons. The 8.334 liters for the walls is 8.334 / 3.785411784 = 2.20 US gallons. A 10 liter order is 10 / 3.785411784 = 2.64 gallons.

Rounding up to tins. For walls and ceiling, 12.334 liters plus 10 percent is 13.57 liters, so any mix of tins adding up to at least that, such as three 5-liter tins (15 liters). In the feet example, 3.43 gallons plus 10 percent is 3.77, so four one-gallon cans.

Things to watch for

• Use the coverage on the tin. It is the single biggest source of error. Premium paints, primers, textured finishes and dark colors can all differ from the 10 m² per liter default.
• Rough or new surfaces drink paint. Bare plaster, new drywall and porous brick soak up the first coat, so a primer or a thinned first coat is usual, and the quantity goes up.
• Light over dark needs more coats. A big color change often takes a third coat. Raise the number of coats rather than guessing at a bigger total.
• Buy about 10 percent extra. Paint is lost in the tray and on the roller, and a spare from the same batch is useful for touch-ups later. The calculator does not add it for you. Tinted paint can vary slightly from tin to tin, so pour tins into one large bucket and stir before you start if you will need more than one.
• Odd rooms need splitting. The model is a plain box. A sloping ceiling, a gable wall or a bay window does not fit it: calculate each part as its own box and add the results. The openings you subtract cannot exceed the wall area, or the calculator reports an error.
• Trim, doors and skirting are separate. Woodwork usually takes a different paint and a small amount. This estimate is for walls and ceiling.
• Everything runs in your browser. The measurements you enter are not sent to a server.

To work out the floor or wall area on its own, use the area calculator.