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How the paint calculator works
In room mode the calculator uses the perimeter of the room: 2 × (length + width) × wall height, which is the total area of the four walls. It then subtracts anything you enter under doors and windows — a standard internal door knocks about 1.8 m² off, a typical window about 1.5 m². Tick the ceiling box and it adds length × width as well.
If you already know the area, switch to the area option and enter it directly; that is handy when you are pricing up a job from a floor plan, or buying paint for a hallway that is not a simple rectangle.
Litres are then worked out as area × coats ÷ coverage. Because paint only comes in whole tins, the calculator then solves the small puzzle of which combination to buy: it finds the smallest total volume that covers the job, and among those options the fewest tins. For a 39.16 m² room at two coats and 11 m²/L you need 7.12 litres, and it will send you out with one 5 L tin and one 2.5 L tin — 7.5 litres — rather than a 10 L tub with 3 litres left over.
Coverage rates in the real world
Tin coverage figures assume a sealed, already-painted surface and a sensible roller technique. Reality is messier. Bare plaster and new plasterboard soak up far more paint on the first coat — expect 20–30% extra, and thin the first coat slightly or use a mist coat of watered-down emulsion. Going from a strong colour to a pale one usually needs three coats, and cheap rollers leave more paint in the tray than they put on the wall.
Two coats is the baseline, not a rule
Most emulsion is specified for two coats, and that is what most rooms need. Add a coat if you are painting over a dark colour, over bare plaster, or if you are changing sheen level from matt to vinyl silk. If you are painting ceilings and walls in different finishes, calculate and buy them separately — they cannot be mixed.
Buying the paint
Check the tin for the coverage figure rather than trusting a rule of thumb, and check the batch codes if you are buying more than one tin of the same colour: tins from different batches can differ very slightly, so mix them together in a large tub before you start if a colour match matters. Keep a small amount of each colour for touching up after the furniture goes back.
Paint FAQs
How much paint do I need for a 5 m × 4.2 m room?
With 2.4 m walls and 5 m² of doors and windows to deduct, the paintable area is 39.16 m². At two coats and 11 m² per litre that is 7.12 litres, so buy one 5 L tin and one 2.5 L tin. At 12 m²/L it falls to 6.53 litres — a 5 L and a 2.5 L tin still, or a single 10 L tub if you want it all in one and do not mind the spare.
Do I need a primer or undercoat?
Bare plaster and new plasterboard should have a mist coat — emulsion thinned roughly 10–20% with water — to seal the surface before full coats. Bare timber, metal and stained walls need a proper primer or stain block first. If you are painting a similar colour over a sound painted wall, you can usually skip straight to two coats.
How many coats over a dark colour?
Plan for three. A dark red or navy under a pale grey will usually show through two coats, especially in raking light. Using a tinted undercoat or a mid-tone basecoat between them often works better than a third full coat of the finish colour.
Does coverage change on a ceiling?
Ceiling paint is formulated to be flatter and is often slightly thinner, so check its own coverage figure — typically 12–14 m²/L. Ceilings also tend to be painted in one or two coats with no cutting-in around as many obstacles, so they usually use slightly less paint per square metre than walls.
How long between coats?
Water-based emulsion is usually touch-dry in 1–2 hours and recoatable in 2–4 hours; solvent-based paints need considerably longer. Cold, damp weather and poor ventilation stretch those times. Check the tin — and remember that painting over tacky paint can drag the first coat off the wall.