Paint calculator: how much paint for a room
Enter the room size, the doors and windows and the coverage on your paint can. You get the paint needed and how many cans to buy.
Free · no signup Runs in your browser
How much paint do I need for a room?
Find the wall area, take off doors and windows, multiply by the number of coats and divide by the coverage on the can. A room 4 m × 3.5 m with 2.4 m walls has 2 × (4 + 3.5) × 2.4 = 36 m² of wall. Take off one door (1.8 m²) and two windows (2.4 m²) to get 31.8 m². Two coats is 63.6 m², and at 10 m² per litre that is 6.36 litres: two 3.78-litre cans. Always use the coverage printed on your own can.
Why we built this calculator
Buying too little paint means a second trip and a risk of a colour that does not quite match; buying too much costs money and leaves cans in the garage. The sums are not hard, but they have several parts: four walls, doors and windows, a ceiling or not, two coats, a coverage figure and a can size. This calculator does all of them, shows each line of the working, and rounds up to whole cans so you know what to put in the basket.
Everything is worked out in your browser. Nothing you enter is sent anywhere.
How it works
The four walls of a rectangular room have a total area equal to the perimeter times the height: 2 × (length + width) × height. Doors and windows are taken off using the area you give for each one, since you know your own sizes better than any default. If you choose to include the ceiling, its area, length times width, is added. The area to paint is multiplied by the number of coats, and by an optional allowance for waste, spills and touch-ups, to get the total area of paint film. Dividing by the coverage, the area one unit of paint covers, gives the volume of paint, and dividing that by the can size and rounding up gives the number of cans. The coverage is something you enter, because it depends on the product and is printed on the label.
How to use it
- Choose metric or imperial. The fields change to metres and litres, or feet and US gallons.
- Enter the room's length, width and height, the number of doors and windows and the area of each.
- Choose the number of coats, and whether to include the ceiling. Add a waste allowance if you like (5 to 10% is common).
- Enter the coverage and the can size from the label of the paint you plan to buy.
- Read the volume and the number of cans, and buy the next whole can.
Use cases
- Buying paint for a bedroom, living room or office
- Comparing the cost of two paints with different coverage
- Planning a ceiling and walls together
- Checking a contractor's quote for paint
- Working out how much extra a second coat needs
- Estimating several rooms by running each in turn
Problems you may run into
Most trouble is about coverage and surfaces.
| What you see | Likely cause | What to do |
|---|---|---|
| I ran out of paint | The surface was rough or porous, or you changed from a dark colour to a light one | Add a waste allowance, and buy primer or an extra coat for dark-to-light changes |
| The coverage on the can is a range | Coverage depends on the surface and the way the paint is applied | Use the lower figure to be safe |
| The result is a fraction of a can | Paint is sold in whole cans | The calculator rounds up; a small leftover is useful for touch-ups |
| The room is not a rectangle | The formula is for a rectangular room | Add up the walls yourself, or run each rectangular part separately |
| Trim, doors and skirting | They are not included | Add them separately; a small can usually covers them |
| Feet and metres mixed | The fields use one unit at a time | Pick the unit first; changing it resets the example numbers |
Getting a good finish
Good results depend on preparation as much as quantity: clean and fill the surface, sand lightly, use primer on bare or very dark surfaces, and keep a wet edge. Two thin coats look better than one thick one. Keep a little leftover paint, with the colour name and finish written on the can, for repairs later.
The formula
How the answer is worked out.
wall area = 2 × (length + width) × heightarea to paint = wall area − doors × area − windows × area (+ length × width for a ceiling)paint film area = area to paint × coats × (1 + waste)paint needed = paint film area ÷ coverage; cans = paint needed ÷ can size, rounded up
A rectangular room is assumed. The default door and window areas and the coverage are examples you can change; use the figures from your own measurements and your paint's label. Metric uses m² per litre; imperial uses square feet per US gallon.
Example: 4 m × 3.5 m room, 2.4 m high
One door, two windows, two coats, 10 m² per litre, 3.78 L cans.
- Walls2 × (4 + 3.5) × 2.4 = 36 m²
- Openings1 × 1.8 + 2 × 1.2 = 4.2 m²; walls to paint 31.8 m²
- Two coats63.6 m²
- Paint63.6 ÷ 10 = 6.36 litres
6.36 litres: 2 cans of 3.78 L
About 1.2 litres are left over.
Terms on this page
- Coverage
- The area one litre or gallon of paint covers in one coat.
- Coat
- One layer of paint over the whole surface.
Common questions
How do I calculate how much paint I need?
Wall area minus openings, times coats, divided by the coverage on the can.
How many coats should I plan for?
Two is usual; a big colour change may need more.
Does it include the ceiling?
Only if you choose it.
How much paint do I need for a bedroom?
It depends on the size; enter your own measurements.
What is coverage?
The area a litre or a gallon of paint covers, printed on the can.
Is anything I enter sent to a server?
No. The calculation happens in your browser and nothing you type leaves your device.
What to work out next
Where these numbers come from
- Wikipedia — Paint What paint is made of and how it dries.
- Wikipedia — Area How areas of rectangles are worked out.
Last verified 2026-10-25 The method on this page is checked against the sources above at least once a year. Spotted something out of date? Tell us and we will fix it.
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