Show the formula and working
This paint calculator tells you how many litres of emulsion you need for a room, and which mix of 5 litre, 2.5 litre and 1 litre tins to buy, for homeowners and renters in the UK, Ireland and the rest of Europe. Enter the room size, doors, windows, coats and the coverage printed on your tin, and the result updates as you type.
Quick answer: the paint calculator multiplies the room perimeter by the wall height, subtracts doors and windows, multiplies by your coats and divides by the coverage on your tin. A 4 m by 3.5 m room with 2.4 m walls, one door and one window needs 5.5 litres for two coats at 12 m² per litre.
What Is a Paint Calculator?
A paint calculator is a tool that converts the size of a room into the number of litres of paint, and tins, needed to cover its walls.
The formula is simple. Add the room length and width, double it to get the perimeter, then multiply by the wall height to get the total wall area. Subtract the area of each door and window. Multiply what is left by the number of coats, then divide by the coverage in square metres per litre. The answer is the litres you need.
Our paint calculator then picks the mix of 5 L, 2.5 L and 1 L tins that covers that amount with the least paint left over, and it shows every step of the sum under the result so you can check it. You can read more about our rounding and testing on the how our calculators work page.
How Do You Use the Paint Calculator?
- Measure the room length and width at floor level and enter them in metres, or switch to feet if you measured in imperial.
- Measure the wall height from the top of the skirting to the ceiling and enter it. Most UK homes are around 2.4 m.
- Enter the number of doors and windows, then check the area of one door and one window. The default door is 1.6 m², which suits a standard UK internal door.
- Choose 1, 2 or 3 coats. Two coats is the normal choice for a fresh colour.
- Type the coverage printed on your tin in m² per litre. The default of 12 is an example only.
- Tick “Paint the ceiling too” if you are using the same paint on the ceiling.
- Read the litres needed, the tins to buy and the paint left over, then open the steps to see the full working.
How Much Paint Do I Need for a Room?
An average UK bedroom of about 4 m by 3.5 m with 2.4 m walls needs around 5.5 litres of emulsion for two coats, which means one 5 litre tin plus a 1 litre tin.
Larger rooms scale up quickly, because the wall area grows with both the perimeter and the ceiling height. A 5 m by 4 m lounge needs about 6.7 litres, but raise the walls to 2.7 m, as in many Victorian and Edwardian houses, and the paint calculator gives 7.6 litres for the same floor size.
A strong colour change, such as dark grey over magnolia, can need a third coat. Bare or patched plaster also drinks paint, so set coats to 3 or lower the coverage figure when the walls are porous.
How Many Square Metres Does a Litre of Paint Cover?
One litre of standard interior matt emulsion covers roughly 10 to 14 m² per coat, but the exact figure is printed on every tin and you should always use that number.
Figures vary by product. Dulux states that its Dulux Matt emulsion covers up to 14 m² per litre, while some contract and trade matts claim up to 17 m² per litre. The word “up to” matters: makers measure on smooth, sealed surfaces, so real walls with filler, texture or old flaking paint usually get less.
That is why the paint calculator asks for your own coverage figure instead of guessing a brand. If you are unsure, use 10 or 11 for a safe estimate. In the EU, tins show the same m² per litre figure, often in several languages, so the input works the same way in Ireland, France or Germany.
Paint Needed for Common UK Room Sizes
The table below was produced with this paint calculator using 2 coats, 12 m² per litre, one door of 1.6 m² and one window of 1.5 m². Use it as a quick check before you measure your own room.
| Room (length x width x height) | Wall area to paint | Paint for 2 coats | Tins to buy |
|---|---|---|---|
| Box room 3 x 1.2 x 2.4 m | 17.06 m² | 2.8 litres | 1 x 2.5 L + 1 x 1 L |
| Small bedroom 3 x 3 x 2.4 m | 25.7 m² | 4.3 litres | 1 x 5 L |
| Double bedroom 4 x 3.5 x 2.4 m | 32.9 m² | 5.5 litres | 1 x 5 L + 1 x 1 L |
| Lounge 4.5 x 4 x 2.4 m | 37.7 m² | 6.3 litres | 1 x 5 L + 1 x 2.5 L |
| Lounge 5 x 4 x 2.4 m | 40.1 m² | 6.7 litres | 1 x 5 L + 1 x 2.5 L |
| Period lounge 5 x 4 x 2.7 m | 45.5 m² | 7.6 litres | 1 x 5 L + 1 x 2.5 L + 1 x 1 L |
Should You Subtract Doors and Windows?
Yes, subtract them, but only for the actual openings: a common UK internal door of 762 mm by 1,981 mm removes about 1.5 m² of wall, and an average window removes 1 to 2 m².
In a small room this makes a real difference. In Sarah’s bedroom below, one door and one window take 3.1 m² off a 36 m² wall area, which saves about half a litre over two coats. For a room with a large bay window or patio doors, measure the width and height of the glass and give the paint calculator that real size as the window area.
Do not subtract fitted wardrobes or radiators unless you will genuinely leave the wall behind them unpainted. Painters usually cut in behind radiators where they can reach.
Does New Plaster Need More Paint?
Yes, new plaster needs an extra thinned mist coat before the normal two coats, so budget for roughly one more coat of paint.
Dulux advises in its guide to painting new plaster that plaster can take up to four weeks to dry, and suggests a mist coat of about ten parts emulsion to one part water. In the paint calculator, set coats to 3 to cover the mist coat. For ceilings on their own, the ceiling paint calculator has a mist coat switch that does this for you.
Worked Example: Sarah’s 4 m by 3.5 m Bedroom
Sarah is repainting the double bedroom of her semi in Leeds from magnolia to a soft sage green. The room is 4 m long, 3.5 m wide and 2.4 m from skirting to ceiling, with one internal door and one window. Her chosen matt emulsion says 12 m² per litre on the tin, and she plans two coats.
- Walls: 2 x (4 m + 3.5 m) x 2.4 m = 36 m²
- Openings: 1 door x 1.6 m² + 1 window x 1.5 m² = 3.1 m²
- Wall area to paint: 36 m² minus 3.1 m² = 32.9 m²
- Paint: 32.9 m² x 2 coats / 12 m² per litre = 5.48 litres, shown as 5.5 litres
- Tins: 1 x 5 L + 1 x 1 L, leaving 0.5 litres for touch ups
If Sarah also paints the 14 m² ceiling with the same emulsion, the paint calculator rises to 7.8 litres and suggests 1 x 5 L, 1 x 2.5 L and 1 x 1 L. To price the job, she can enter 32.9 m² into the paint cost calculator with her local tin price.
Common Mistakes When Estimating Paint
- Using floor area instead of wall area, which is the most common reason people get a different answer from a paint calculator. A 14 m² floor has 36 m² of wall at 2.4 m high.
- Forgetting the second coat, which doubles the paint needed.
- Trusting the best case coverage on a rough or patched wall.
- Buying tins from different batches of a mixed colour. Stir them together in a bucket before you start so the shade matches.
- Sanding old paint in a pre 1960s home without checking for lead. The HSE notes that lead pigments were widely used in household paint until the 1960s, so wet sand and never burn off old layers.
Keep windows open while you paint and while the room dries. Water based emulsions are low odour but still need ventilation. Use a stable stepladder for the tops of walls, and move it instead of overreaching.
Paint Calculator FAQs
How much paint do I need for a 3 x 3 metre room?
A 3 m by 3 m room with 2.4 m walls, one door and one window has about 25.7 m² of wall. At 12 m² per litre and two coats, that is about 4.3 litres, so a single 5 litre tin is enough for the walls.
How many coats of paint does a room need?
Most rooms need two coats for an even finish, which is the default in the paint calculator. A third coat helps when covering a much darker colour or patchy filler, and new plaster needs a thinned mist coat first. Set the coats to match your job.
Is 5 litres of paint enough for a bedroom?
Five litres covers about 60 m² at 12 m² per litre, so it gives two coats on roughly 30 m² of wall. That suits a small double bedroom. The paint calculator gives 5.5 litres for a 4 m by 3.5 m room, so add a 1 litre tin.
Does the paint calculator include the ceiling?
Only if you tick the ceiling option. It then adds length times width to the wall area. Ceilings usually take a different white matt, so many people use the separate ceiling tool and buy the two paints separately.
Should I buy extra paint?
The tool already rounds up to whole tins and shows what is left over. Keeping half a litre or so is useful for touch ups, and buying it all at once keeps a mixed colour consistent across every tin.
Can I use the paint calculator for exterior walls?
You can, but outside walls are usually measured by elevation rather than room perimeter, and rough render uses far more paint. The masonry paint calculator handles surface texture, so it gives a more realistic answer for brick, render or pebbledash.
Checked October 2026. Sources: Dulux product data and Dr Dulux new plaster guide, Health and Safety Executive (HSE).