Running out of paint halfway through a second coat is a miserable experience, and the shop may not match the batch. The calculation takes five minutes and needs only a tape measure and the coverage figure printed on the can.
Step 1: wall area
Wall area = room perimeter x ceiling height. For a 4 m x 5 m room with 2.7 m ceilings, the perimeter is 4 + 5 + 4 + 5 = 18 m, and wall area is 18 x 2.7 = 48.6 square metres. Include every wall you will paint. If you are painting the ceiling too, add the floor area, 20 square metres here, as a separate line, since ceilings often need a different paint.
Measure each wall separately if the room is not a rectangle. Alcoves, chimney breasts, and sloped ceilings are easier as separate rectangles added together than as one clever formula.
Step 2: subtract doors and windows
Subtract about 2 square metres per standard door and about 1.5 square metres per average window. Our room has one door and one window: 48.6 – 2 – 1.5 = 45.1 square metres of paintable wall. For exactness, measure each opening’s width x height instead of using the rules of thumb, especially for large windows or double doors.
Do not subtract trim, skirting, or radiators unless you are genuinely not painting around them; the paint used cutting in around them roughly cancels the area saved.
Step 3: coats and coverage
Multiply by the number of coats, usually 2 for a colour change: 45.1 x 2 = 90.2 square metres to cover. Divide by the coverage on the can, often around 10 square metres per litre: 90.2 / 10 = 9.02 litres. Add 10% for waste, roller absorption, and touch-ups: about 10 litres. So buy a 10-litre can, or two 5-litre cans, rather than 9 litres exactly.
Coverage varies by paint type and surface: bare plaster drinks paint and may need a mist coat (watered-down first coat), while previously painted walls in good condition hit the stated coverage. Dark colours over light walls sometimes need a third coat, so budget for it if the change is drastic.
Litres, gallons, and buying
In the US, coverage is often stated as square feet per gallon, around 350 square feet per gallon. The method is identical: paintable area in square feet, times coats, divided by 350, rounded up, plus waste. Our 45.1 square metres is about 485 square feet, so two coats need 970 / 350 = 2.77 gallons, plus waste, about 3 gallons. Buy the next size up when you are between sizes; leftover paint in a sealed can keeps for touch-ups.
Primers and mist coats
New plaster drinks paint, so the first coat should be a mist coat: ordinary emulsion thinned with roughly 10 to 20% water, which soaks in and seals the surface. Without it, the top coats peel or need extra coats to cover. A mist coat covers roughly the same area per litre as normal paint, so include it in your quantity calculation as an extra coat.
Stains, knots, and patched areas need a stain-blocking primer before the top coats, or the mark bleeds through within weeks. Primer is not a substitute for paint and paint is not a substitute for primer: each does a job the other cannot. Budget primer separately, since it comes in different cans and coverage figures.
Try it faster with the tool
You can follow every step above by hand, but the free Paint Calculator — Litres and Gallons for a Room, With Doors, Windows and Coats does the arithmetic for you. Enter your numbers and it gives the answer right away, so you can check your hand calculation or skip straight to the result.
Quick tips
- Wall area = perimeter x height.
- Subtract doors and windows, then multiply by coats.
- Add 10% waste and round up to the next can size.