By the Home Savings Calculator team · 29 August 2026 · 5 min read
Add the length of all four walls to get the perimeter, then multiply by the ceiling height. That is your wall area. For a 4m × 3m room with a 2.4m ceiling: (4+3+4+3) × 2.4 = 33.6 m².
Most decorators do not subtract doors and windows. Leaving them in builds useful slack for touch-ups and the paint a roller leaves behind in the tray. Subtract them only if they take up an unusually large share of the wall.
Put that wall area into the paint calculator. It converts area and coats into litres, then into the tin sizes you would actually buy, and prices it at your chosen quality.
Litres needed = wall area × coats ÷ coverage per litre. Coverage is printed on every tin, typically around 10–14 m² per litre for emulsion on a sealed wall.
Treat the tin figure as a best case. Bare plaster, textured surfaces and deep colours all drink more, and coverage varies enough between products that it is worth checking the specific tin rather than assuming a universal number.
Two is standard over a similar colour. Go to three when painting light over dark, over a strong colour, or when using a thin premium paint with high pigment but low build.
This is where better paint sometimes pays for itself: a paint that covers in two coats where a budget one needs three saves you a tin and a day. The calculator lets you compare coats and quality side by side to see whether that holds for your room.
Primer or a mist coat is needed on bare plaster, new plasterboard, bare wood, and when covering a strong colour. Fresh plaster in particular will drink an undiluted topcoat and leave a patchy finish.
Primer is costed separately in the calculator, because whether you need it is a property of your walls rather than your paint choice.
Paint comes in 2.5L and 5L tins, so the practical question is which combination to buy rather than the exact litre figure. Rounding up matters: 7.2 litres means two 5L tins or one 5L plus one 2.5L, not 7.2 litres.
If the decorating is part of a bigger job, price the whole room — paint and flooring together — in the room renovation cost calculator, and read budgeting a renovation before you commit to a number. Choosing the floor at the same time? Start with flooring costs per m².
