Most paint disasters are not painting disasters — they are measuring shortcuts taken before the tin was opened. Five careful minutes with a tape measure gives you a wall area you can trust, a paint order that covers the job in one batch, and no half-painted wall while a second tin is mixed to a slightly different shade.
Measure the four walls, not the floor plan
Measure each wall's length at skirting level and multiply the total perimeter by the ceiling height. Floor plans from estate listings are a rough guide only; they routinely exclude alcoves, chimney breasts and the odd angles of older rooms. Write each wall separately in your notes — if two opposite walls disagree, use the larger figure and find out why they differ before you paint, because out-of-square rooms need the extra paint at the wide end.
Deduct openings the standard way
A standard interior door with its frame accounts for about 21 square feet, and an average window about 15 square feet. Deducting them stops you over-buying in small rooms where openings can be a fifth of the wall. Do not deduct small windows, and never deduct for radiators — you will paint around them, not remove the wall behind them.
Decide coats before you calculate
A like-for-like colour refresh over sound paint may take one good coat. Everything else — colour changes, new plaster, patched walls — takes two finish coats, and new plaster needs a mist coat or primer before those. Count primer as its own product and pass; it never replaces a finish coat.
Round up, and record the batch
Always round the final figure up to a whole tin, and where two tins are needed, mix them together in one bucket ('boxing' the paint) so any tiny batch difference disappears. Write the colour name, code and room on the leftover tin with a marker — your future self, touching up a scuff in three years, will be grateful.
Frequently asked questions
Do I measure in feet or metres?
Either, as long as you are consistent — the calculator on this site works in feet. If you measured in metres, multiply each measurement by 3.281 before entering it.
How do I handle a room with a sloping ceiling?
Measure the wall as a rectangle up to the highest point, calculate the triangular part separately (base x height / 2), and add the two areas together.