Paint calculator: rooms, walls, ceilings and trim

Quick answer

A 12 × 12 ft room with 8 ft walls, one door and one window has about 349 sq ft of wall. Two coats at 350 sq ft per gallon need 2.0 US gallons; with the ceiling, 2.8 gallons, so buy 3. One gallon of primer at 300 sq ft per gallon primes about 300 sq ft in one coat. The table below lists more room sizes.

What this calculator does

It estimates interior paint for four kinds of job. Room walls: the four walls of a rectangular room, less doors and windows, with the ceiling if you want it. One wall: a single wall, less its openings, for an accent wall. Ceiling only: length times width. Doors and trim: both faces of each door plus baseboards, skirting or door casings, by their length and face height.

For each job it applies the number of coats and the coverage of the product and rounds up to whole cans. Choose Primer to switch the example coverage and the number of coats to primer values; replace them with the values on your can.

It does not include exterior paint, textured surfaces or wastage in the tray and roller beyond the rounding to whole cans.

How to use it

1. Choose metric or US units.
2. Under What to paint, choose Room walls, One wall, Ceiling only or Doors and trim.
3. Enter the sizes the form asks for: length and width of the room, the wall height, the width of a single wall, or the number of doors and the length and face height of the trim.
4. For walls, enter the number of doors and windows and the area of one of each. Set the number to 0 to ignore them.
5. Choose Paint or Primer. Then check the number of coats, the coverage and the can size against the label of your product.
6. Select Calculate.

How the result is calculated

1. Room walls: area = 2 × (length + width) × height − openings, plus length × width if the ceiling is included.
2. One wall: area = wall width × height − openings. Ceiling only: area = length × width.
3. Openings = doors × area of one door + windows × area of one window.
4. Doors and trim: area = doors × 2 faces × area of one door + trim length × face height.
5. Paint needed = area × coats ÷ coverage.
6. Cans = paint needed ÷ can size, rounded up to the next whole can.

The amount of paint is shown rounded up to 0.1 litre or 0.01 US gallon. The calculator adds no allowance of its own: the rounding to whole cans is the reserve.

Worked example

A room is 4 m long and 3 m wide with walls 2.4 m high. Wall area: 2 × (4 + 3) × 2.4 = 33.6 m². One door of 1.9 m² and one window of 1.4 m² take away 3.3 m², which leaves 30.3 m².

Two coats cover 30.3 × 2 = 60.6 m². With a coverage of 12 m² per litre, the paint needed is 60.6 ÷ 12 = 5.05 L, shown as 5.1 L. In cans of 2.5 L: 5.05 ÷ 2.5 = 2.02, so the result is 3 cans.

This example shows why the amount matters as well as the number of cans. Two cans hold 5 L, which is only 0.05 L less than the estimate. The calculator still rounds up, because it cannot know how much your walls absorb. When the result is this close, look for a smaller can to cover the difference.

In US units: a room of 12 ft by 10 ft with 8 ft walls has 2 × (12 + 10) × 8 = 352 sq ft of wall. Less a door of 20 sq ft and a window of 15 sq ft, 317 sq ft remain. Two coats at 350 sq ft per gallon need 634 ÷ 350 = 1.82 US gallons, so you buy 2 one-gallon cans.

Doors and trim: two doors of 20 sq ft are painted on both faces, 80 sq ft, and 44 ft of baseboard with a 3 1/4 in face adds 44 × 3.25 ÷ 12 = 11.9 sq ft, together 92 sq ft. Two coats at 350 sq ft per gallon need 0.53 gallons: one gallon can, because two quarts hold only 0.5 gallons.

Primer: the walls of the same room, 317 sq ft, primed with one coat at 300 sq ft per gallon need 1.06 gallons, so 2 cans of one gallon, or one gallon and a quart.

Paint for common room sizes

The tables use the calculator with the example values: walls 8 ft or 2.4 m high, one door and one window of the example sizes, two coats, 350 sq ft per US gallon or 12 m² per litre. The paint is rounded up to 0.01 gallon or 0.1 L; round up to the cans you buy. With other heights, openings or paint, use the calculator.

The metric rooms are close to, not exact conversions of, the US sizes: 3.6 m is 11.8 ft.

About the example values

Coverage differs from paint to paint, so the value on your can is the one to use. The examples are taken from what manufacturers publish.

The calculator starts with 12 m² per litre and 350 sq ft per US gallon, the lower ends of these figures. Ceiling paints state similar or higher figures: Benjamin Moore states 400 to 450 sq ft per gallon for its Waterborne Ceiling Paint and Sherwin-Williams 350 to 400 for Eminence ceiling paint; Dulux states 16 m² per litre for Ceiling White in Australia.

Primer covers less than paint. The primer presets are 300 sq ft per gallon, the lower end of the 300 to 400 that KILZ states for its Original and General Purpose primers, and 8 m² per litre, the lower end of the 8 to 10 that Dulux Trade states for its plaster primer in the UK, with one coat. Zinsser states 450 sq ft per gallon for its Bulls Eye 1-2-3 on non-porous surfaces but 350 on porous ones, so new drywall and bare plaster use more.

For trim, the example face heights are 3 1/4 in, a baseboard size sold in the United States and Canada, and 119 mm, an MDF skirting sold by UK merchants such as Wickes and B&Q. Enamel for doors and trim covers about as much as wall paint: Sherwin-Williams states 350 to 400 sq ft per gallon for ProClassic semi-gloss. Dulux UK says a 750 ml tin is enough for a door.

The example areas of 20 sq ft for a door and 15 sq ft for a window are the averages that one US manufacturer, McCormick Paints, uses in its own guide. The metric examples of 1.9 m² and 1.4 m² are the same areas converted. Measure your own doors and windows if they are larger or smaller.

A US gallon is 3.785 litres. It is smaller than the imperial gallon of 4.546 litres that was used in the United Kingdom and Canada. This calculator uses the US gallon.

Common mistakes

• Forgetting the second coat. The calculators of Dulux and Benjamin Moore, for example, assume two coats.
• Using the coverage of a different product. Sherwin-Williams, for example, states 200 to 300 sq ft per gallon for primer, less than for its wall paint.
• Measuring the floor instead of the walls. Wall area depends on the height of the room.
• Subtracting built-in wardrobes or tiled areas without measuring them. Enter them as additional windows of the right area, or leave them in as a reserve.
• Buying cans of the same colour from different batches for one wall. Mix the cans in a larger bucket before you start if you need more than one.