How Much Paint Do I Need? UK Guide
Published 12 August 2026 · Browse all tools
Standing in the paint aisle guessing how many tins to grab is how you end up with either half a wall unpainted on a Sunday evening or a cupboard full of tins slowly skinning over. The maths is simple once you know the two numbers that matter: the area you are covering, and how far a litre of your paint actually goes. This guide walks through both for UK rooms, works a real example end to end, and points out the two things most people get wrong — the coverage figure and the mist coat.
To turn your room size straight into litres and tins, use the Paint Coverage Calculator (UK). This guide explains the numbers behind it so you can sanity-check the result.
Use the Paint Coverage Calculator (UK)Start with the area you are actually painting
Work out the wall area first. Multiply the room perimeter by the wall height: for a 4 m by 3.5 m room, the perimeter is 2 x (4 + 3.5) = 15 m, and at a 2.4 m ceiling that is 36 m² of wall. Then take off the openings. A standard UK internal door is about 1.8 m² once you allow for the frame, and a typical window is around 1.4 m², so one of each knocks 3.2 m² off, leaving 32.8 m² of paintable wall. If you are doing the ceiling too, that is simply length times width, so 4 x 3.5 = 14 m² on top.
Takeaway: paintable wall area is perimeter times height, minus about 1.8 m² per door and 1.4 m² per window.
The number that trips everyone up: coverage rate
Here is where most estimates go wrong. The tin will often say something like 16 or 17 m² per litre. That is a laboratory figure measured on a smooth, sealed, roller-friendly board. On a real wall you are cutting in with a brush, losing paint into the roller and the tray, and feeding a surface that absorbs more than a test panel. The honest working figure for standard matt emulsion is more like 10 to 12 m² per litre. Coverage and opacity are classified under BS EN 13300, the standard for interior water-borne coatings, but for buying purposes the number to trust is the on-site one, not the headline on the label. Use 12 m² per litre for emulsion and you will not be caught short.
Takeaway: budget 10 to 12 m² per litre for emulsion, not the best-case figure printed on the tin.
How many litres per square metre?
If a litre covers 12 m², then one square metre needs about 0.083 litres per coat, so roughly 0.08 to 0.10 litres per m² per coat is a safe planning figure. Two coats on that 32.8 m² wall is 65.6 m² of coverage, which at 12 m² per litre is 5.47 litres. Add ten percent for wastage and you are at about 6 litres. That is the whole calculation: area, times coats, divided by coverage, plus a bit for waste.
Takeaway: multiply area by coats, divide by 12, add ten percent — that is your litres.
Worked example: painting a Nottingham bedroom
Take a real job: repainting a bedroom in a Nottingham semi, 4.0 m by 3.5 m, 2.4 m walls, one door and one window, two coats of standard matt emulsion, walls only.
The wall area is 15 m of perimeter times 2.4 m, which is 36 m². Subtract 1.8 m² for the door and 1.4 m² for the window and you have 32.8 m² of paintable wall. Two coats means 65.6 m² to cover. At 12 m² per litre that is 5.47 litres, and with ten percent wastage it comes to 6.01 litres.
So you need about six litres. The sensible buy is a 5 litre tin plus a 2.5 litre tin, which gives you 7.5 litres — enough to finish comfortably and keep a labelled tin back for touch-ups. Buying two 5 litre tins wastes nearly a whole tin; buying a single 5 litre tin leaves you a litre short with the second coat half done. If you were painting the 14 m² ceiling as well, two coats adds about 2.6 litres with wastage, so a 2.5 litre tin plus a 1 litre top-up covers it.
Takeaway: for this room, about 6 litres of wall paint — buy a 5 litre and a 2.5 litre rather than rounding to two big tins.
Don't forget the mist coat on new plaster
Fresh plaster is thirsty. Paint it straight with full-strength emulsion and the plaster drinks the first coat unevenly, leaving patches that flash and can peel later. The fix is a mist coat: contract matt emulsion thinned with around 10 to 20 percent water, brushed and rollered on as a sealing first layer before your two proper coats. This is standard practice and is set out in BS 6150, the code of practice for painting buildings. Allow roughly an extra coat's worth of paint for it — a thinned coat spreads further, around 10 m² per litre. People who skip this step are the ones repainting the same wall twice.
Takeaway: on bare plaster, add a thinned mist coat before your two top coats, or the finish will flash and peel.
When two coats is not enough
Two coats is the default for walls and ceilings, but the room decides. Going from a dark feature wall to a pale colour, covering a bold red, or painting over patchy filler and repairs can all need a third coat to look even. Strong colours with weak pigments — some yellows and reds — are notorious for needing an extra coat regardless of the surface. If in doubt, buy for three coats on the tricky wall and two everywhere else; the leftover is your touch-up stock. A 2026 price check at a merchant like Screwfix or Wickes puts trade white contract matt at a few pounds a litre, so an extra tin is cheap insurance against a wasted afternoon.
Takeaway: budget a third coat for big colour changes and strong pigments, not just for new plaster.
What decorators do differently
In practice, a working decorator does not measure every wall to the centimetre for a normal room. They will pace the perimeter, use a standard 2.4 m height, and round up to whole tins, because the wastage margin swallows the small stuff. What they do not skimp on is the mist coat on new plaster and the cutting-in allowance, and they always keep the offcut tin from each colour, labelled with the room, for the inevitable scuff a year later. They also decant into a kettle and only work from that, so a whole 5 litre tin is not sitting open drying out while they cut in around a whole room.
Takeaway: round up to whole tins, never skip the mist coat, and keep a labelled touch-up tin for every colour.
Work out your paint
Put your room size into the Paint Coverage Calculator (UK) to get litres and a tin combination in one go, with doors, windows, ceilings and mist coats handled for you. If you are decorating the rest of the room, the Wallpaper Calculator (UK) works out rolls by drop and pattern repeat, and the Tile Calculator handles wall and floor tiles with wastage.
Disclaimer: This guide is for planning. Real coverage varies with surface texture, porosity, colour change and the specific product, so always check the figure on your tin and buy a little over.