Free tool
Paint calculator
Wall area, minus the holes, times the coats, divided by the spread rate.
Paint coverage is quoted on the tin as a spread rate in square metres per litre, and that figure assumes a sealed, smooth, similarly coloured surface. Most of the reasons people run out come from the surface rather than the arithmetic.
The sum
Add up the length of every wall you are painting, multiply by the height, take off the doors and windows, multiply by the number of coats, and divide by the spread rate on the tin.
A standard internal door is about 1.6 m² and an average window about 1.5 m², so a room with one door and two windows is around 4.6 m² of openings. Being precise about that matters less than being roughly right, because the deductions are small next to the wall area.
What eats more paint than the tin says
- Bare plasterboard. Drinks the first coat. Seal it first and the topcoats then behave like the tin says.
- Render, brick and texture. More surface than the flat measurement suggests, sometimes half as much again.
- A dark colour going lighter. Three coats, sometimes four, and a tinted undercoat saves a topcoat.
- Roller nap and technique. A thick nap holds more paint and lays more of it on.
Undercoat and sealer are not counted above. If the surface needs them, work them out as a separate quantity at their own spread rate.
Ceilings
A ceiling is length times width and it takes about the same per square metre as a wall, sometimes a little more because ceiling white is usually flat and applied generously. Add it as a separate calculation rather than folding it into the wall figure.
Buy slightly over, in one batch
Paint is tinted in batches and two tins of the same colour mixed weeks apart can differ enough to see on a long wall. Buying the whole job at once from the same batch is worth more than saving the price of a half tin, and the leftover is what you touch up with in three years.
Getting a painter instead
A quote from a painter should separate preparation from painting, because preparation is where the hours actually go and where quotes diverge most. Two quotes that look far apart are often quoting different amounts of filling and sanding rather than different rates. The job cost calculator is useful for pulling the labour apart from the materials.
Common questions
How much paint do I need for a room?
Add the length of the walls, multiply by the height, subtract the doors and windows, multiply by the coats, and divide by the spread rate on the tin. A typical 4m by 3m room with 2.4m ceilings is about 34 m² of wall, so around 4 litres for two coats at 16 m² per litre.
How many square metres does a litre of paint cover?
Around 16 m² per litre is typical for a topcoat on a sealed, smooth wall, but the figure is on the tin and it varies by product. Bare plasterboard, render, texture and dark to light colour changes all reduce it, sometimes considerably.
How many coats of paint do I need?
Two over a sealed surface in a similar colour. Three when you are going from dark to light, painting bare plasterboard without a sealer, or using a strong colour with poor opacity. A tinted undercoat often saves a topcoat and costs less than one.
Does the calculator include undercoat?
No. It works out topcoat only. Sealer and undercoat have their own spread rates and are usually a single coat, so calculate them separately using the same wall area.
Should I buy all the paint at once?
Yes. Tinted paint is mixed in batches and two tins of the same colour tinted weeks apart can differ enough to show on a long wall. Buy the job in one go and keep the leftover for touch ups.