Worldwide
Paint calculator
Buying paint by guesswork ends one of two ways: a half-finished wall when a tin runs dry, or a shelf of part-used cans that skin over before the next job.
Enter the total wall area in square metres; if you only have room dimensions, multiply the perimeter (add up the wall lengths) by the wall height.
This calculator removes the guess by turning a few measurements into a litre figure. It starts from the total wall area you plan to paint, takes off a fair allowance for the doors and windows you cut around, multiplies by the number of coats, and divides by how far a litre of your paint actually spreads. The result is the litres needed, the paintable area once the openings come out, the area covered across all the coats, and the litres each coat uses. Wall area is the figure most people lack, so here is the quick way to get it: add up the length of every wall you are painting to get the perimeter, then multiply by the height of the room. A 4 by 5 metre room with 2.4 metre walls has an 18 metre perimeter and so about 43 square metres of wall before openings. Coverage is the other number that matters, printed on the tin as a spreading rate in square metres per litre. Smooth, previously painted plaster takes paint thinly and covers well, around 11 to 13 square metres a litre for matt emulsion. Bare plaster, fresh filler, textured or very dark walls drink far more, so the same litre covers less and you may need a thinned mist coat first. Two coats is the normal minimum for an even finish, and a light shade over a dark wall can want three. People reach for this before a decorating job, when pricing up a rental refresh, or when matching how much primer, undercoat and topcoat to buy in one trip. Because real walls, rollers and painters vary, treat the litres as a confident shopping figure with a little spare, not a measurement to the millilitre.
How it works
- Work out the wall area: add the wall lengths for the perimeter, then multiply by the room height.
- Enter that area in square metres in the wall area field.
- Set how many doors and windows you paint around; each is taken off as a standard opening.
- Choose the number of coats, usually two for an even finish.
- Enter the coverage from the tin in square metres per litre, then read off the litres needed.
litres = (wall area - openings) x coats / coverage
The paintable area is the wall area less an allowance for each door and window, since those are not painted with wall paint. Multiplying by the number of coats gives the total area the roller actually covers, because each coat repaints the same wall. Dividing by the coverage, the square metres one litre spreads on a single coat, converts that area into a volume of paint. Every term is a plain multiply or divide, so doubling the coats doubles the paint and a higher coverage figure lowers it.
- wall area
- total area of the walls, perimeter times height, in square metres
- openings
- 1.8 m² per door and 1.5 m² per window, taken off the wall area
- coats
- number of times each wall is painted
- coverage
- square metres one litre covers on a single coat
Typical paint coverage by surface
| Smooth, previously painted plaster | about 11 to 13 m² per litre | matt emulsion, the easy case |
| Bare or new plaster | less per litre, plus a mist coat | a thinned first coat soaks in |
| Textured or woodchip walls | roughly 8 to 10 m² per litre | the texture holds more paint |
| Dark colour being covered | add a coat or use primer | light over dark needs more |
Worked example
A room has 40 m² of wall, one door, two windows, two coats, and a paint that covers 11 m² a litre: the openings take off 4.8 m² (one door at 1.8 and two windows at 1.5), leaving 35.2 m² to paint. Two coats means 70.4 m² of painting, and at 11 m² a litre that needs 6.4 litres, or 3.2 litres a coat. Round up to a 5 litre tin plus a small one and you have enough with a little spare. Drop to one coat over an already similar colour and it halves to 3.2 litres.
Key facts
- Paint is worked out by area and coats, not by the number of walls or rooms.
- Two coats is the normal minimum for an even, durable finish.
- Coverage on the tin is per coat, so two coats halves the effective spread.
- Bare plaster needs a thinned mist coat that the smooth-wall figure does not include.
Tips
- Measure the perimeter and height once and keep the wall area noted for future redecorating.
- Buy all the paint for one wall from the same batch so the colour matches exactly.
- Keep a labelled jar of leftover paint for touch-ups rather than buying a fresh tin later.
- For a big colour change, a tinted primer can save a topcoat and give a truer final shade.
Frequently asked questions
How do I measure the wall area?+
Add the length of each wall you plan to paint to get the perimeter of the room, then multiply by the wall height. For a room 4 m by 5 m with 2.4 m walls, the perimeter is 18 m and the wall area is about 43 m² before you take off doors and windows. You do not need to be exact to the centimetre.
What coverage figure should I use?+
Use the spreading rate on the tin, given in square metres per litre. Matt emulsion on smooth painted plaster is often 11 to 13 m² a litre. Silk and durable finishes can differ, and primers and bare-plaster mist coats cover less, so check the specific product rather than assuming one number.
Why does the tool subtract doors and windows?+
You do not paint the glass or the door leaf with wall paint, so leaving them in would overstate the area. The tool takes off 1.8 m² for each door and 1.5 m² for each window, which are typical sizes. If your openings are much bigger, such as patio doors or a large picture window, measure them and lower the wall area yourself.
How many coats do I need?+
Two coats is the usual minimum for an even, hard-wearing finish on a painted wall. One coat can do for a light refresh in the same colour. Going lighter over a dark wall, or using a strong colour, often needs a third coat or a tinted primer first, so set the coats to match the job.
Should I buy exactly the litres shown?+
Round up to whole tins and keep a little spare. Paint is sold in fixed sizes, often 2.5 and 5 litre tins, and you want enough to finish without a second shop trip, plus a small amount kept back for touch-ups. Buying a touch over the calculated figure is normal and sensible.
Does this cover ceilings and woodwork?+
It covers walls. A ceiling is its own area (room length times width) and usually wants ceiling paint, so calculate it separately and add it. Skirting, frames and doors use gloss or satinwood and far less paint, so estimate those on their own rather than through the wall figure.
Things to watch
- The door and window allowances are typical sizes; unusually large openings need adjusting by hand.
- Coverage varies a lot by product and surface, so trust the tin over any single rule of thumb.
- This estimates walls only; ceilings and woodwork are separate areas with their own paints.
Last updated: 2026
This is an estimate for general guidance, not financial, tax, legal or medical advice. Figures can change and individual circumstances vary. Always confirm with an official source before making decisions.
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