About the Paint Calculator
Paint Calculator estimates how much paint you need for a room so that you buy enough without wasting money. Enter the length, width and wall height in metres or feet, the number of doors and windows, any other area to leave out, and whether to paint the ceiling. Then set the number of coats, the coverage printed on the tin and how much extra to allow for waste. The tool works out the wall and ceiling area, takes away the openings, and shows the litres or US gallons needed with the number of cans of your chosen size. It also shows each area figure so you can double-check. Use it before you go to the shop, and everything is worked out in your browser.
Updated September 2026
How to use the Paint Calculator
- 1Choose metres or feet.
- 2Enter the room's length, width and wall height.
- 3Enter the number of doors and windows, and any other area to leave out. Tick the box to include the ceiling.
- 4Set the number of coats, the coverage from the tin, the waste allowance and your can size.
- 5Read the paint needed and the number of cans, and check the areas beneath.
Measuring the room
Measure the length and width of the floor and the height from floor to ceiling. Tape measures and phone apps both work, and rounding up to the nearest ten centimetres is fine. For an odd-shaped room, split it into rectangles and add the results, or measure each wall as a separate room with a very small width.
Count every door and window that will not be painted. If a window is much bigger than usual, such as patio doors, add its area to the other area box so the estimate stays accurate.
Getting the inputs right
- Coverage: copy it from the tin, and check that it is for one coat.
- Coats: two is standard for walls, more for dramatic colour changes.
- Waste: 10 percent is a good default, more on rough or porous walls.
- Can size: choose the size you will buy, since paint comes in fixed sizes.
Buying the paint
The result shows the exact amount and the whole cans you need, rounded up. It often helps to buy one larger can rather than several small ones, because you mix the colour once and avoid slight differences between batches. Keep a little left over for touch-ups and store it in a sealed can.
Light colours over dark walls, new plaster and drying stains all need a primer or an extra coat, so allow for them. Lower quality paint covers less, so a better paint can be cheaper in practice if it saves a coat.
Related tools
Area Calculator finds the area of other shapes, Unit Converter changes metres to feet and litres to gallons, Percentage Calculator handles waste and discounts, Volume Calculator helps with tanks and boxes and Concrete Calculator plans slabs and footings for outdoor projects.
Features
- Metres or feet, litres or US gallons
- Walls, optional ceiling and openings taken off
- Doors and windows counted at standard sizes
- Any number of coats up to six
- Coverage and waste allowance you can change
- Number of cans of your chosen size
- Area figures shown for checking
- Runs in your browser
Frequently asked questions
How do I calculate how much paint I need?
Work out the area to paint, multiply it by the number of coats and divide by the coverage of the paint. A 5 by 4 metre room with 2.5 metre walls has 45 square metres of wall. Take off a door and two windows and about 40 square metres remain. Two coats at 10 square metres per litre needs about 8 litres.
How is the wall area worked out?
Add the length and width, double the result to get the distance around the room, and multiply by the wall height. The tool then subtracts a standard door of 0.9 by 2 metres for each door and a window of 1.2 by 1.2 metres for each window. Use the other area box for bigger openings.
What coverage should I use?
Use the figure printed on the tin. Most emulsion and wall paints cover 8 to 12 square metres per litre for one coat, or about 300 to 400 square feet per US gallon. Textured or dark walls, and cheap paint, may give less.
How many coats do I need?
Two coats give an even finish on most walls. One coat may be enough over a similar colour with good paint, and three may be needed for a strong colour change or over dark colours. A primer first helps on bare plaster.
Why add extra for waste?
Paint is lost in the tray, on rollers and brushes and in tin corners, and rough or porous walls soak up more. The tool adds 10 percent by default. Add more for very rough walls and touch-ups later, and check that the shop has the same batch in case you need more.
Should I paint the ceiling with the same paint?
Often not, because ceilings usually take a special flat matt paint. Tick the ceiling box and the tool adds the ceiling to the total and shows the ceiling area on its own line, so you can divide that area by the coverage to see how much ceiling paint you need.
Can I use it for the outside of a house?
It is designed for rooms, but you can use it for outside walls by entering each wall's length and height as a rectangle and adding the results. Exterior paint has its own coverage figure and rough surfaces use more, so follow the tin.