Subtracts doors and windows, handles multiple coats, and lets you set the coverage rate your tin actually claims.
Paint needed
8.73 L
39.3 m² over 2 coats at 9 m²/L
Buy
2 × 5L
1.27 L spare
Wall area, gross
43.2 m²
Doors and windows
−3.9 m²
Wall area, paintable
39.3 m²
Total across 2 coats
78.6 m²
Coverage is the biggest variable and is not a constant. US brands quote 350–400 sq ft per gallon and Behr as little as 250 on difficult surfaces; UK brands often quote 12–14 m² per litre, which is roughly 1.5 times the American figure for the same wall. Check the tin and change the field above rather than trusting a default.
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About the Paint Calculator
Working out how much paint a room needs is simple geometry divided by one number, and that one number is where all the uncertainty lives.
The geometry is unarguable: perimeter times wall height, minus the doors and windows, times the number of coats. The divisor is the coverage rate, and it is not a property of paint in the way that density is a property of water. It is a manufacturer claim about how far a litre goes on an assumed surface, and different manufacturers assume different surfaces.
That is why the coverage field on this page is editable rather than fixed. Most calculators bury a house figure and give you one answer. The honest version asks what your tin actually claims, because the gap between a conservative figure and an optimistic one is the difference between buying one tin and two.
Mathematical Formula & Logic
Paintable wall area:
A = 2 x (length + width) x height − (doors + windows)
Add the ceiling, length x width, only if you are painting it. Openings come off the wall and never off the ceiling.
Paint volume:
V = A x coats / coverage rate
Both terms on top are linear, so a third coat adds exactly 50% to a two-coat estimate. Coverage divides, which means an optimistic rate understates your requirement in direct proportion — a claim 1.5 times too high leaves you with two thirds of the paint you needed.
What the coverage number actually is
Manufacturers publish these figures and they do not agree, because they are measured against different assumed surfaces:
Sherwin-Williams quotes 350 to 400 square feet per US gallon for one coat.
Glidden quotes up to 400, specifying primed, smooth, non-porous surfaces.
Benjamin Moore quotes 400 for its one-coat products.
Behr states 250 to 400, the lower end covering porous, rough, textured or previously unpainted work.
So the American range alone spans a factor of 1.6. None of these is wrong; they describe different jobs.
The conversion that catches people out
UK and European tins quote square metres per litre, and the figures are markedly more generous. A common claim of 13 m² per litre converts to about 530 square feet per gallon — roughly 1.51 times the 350 an American tin claims for the same wall.
Using the exact definitions, 1 square foot is 0.09290304 m² and 1 US gallon is 3.785411784 litres. That is not a rounding difference or a mistake by either party. It reflects genuinely different assumptions about substrate and application. But if you take a figure from one market and apply it to a tin from the other, you will be wrong by half again.
Step-by-Step Example
A bedroom 5 m long, 4 m wide, with 2.4 m walls, one door and two windows. Two coats.
1. Gross wall area: 2 x (5 + 4) x 2.4 = 43.20 m².
2. Openings: one door at 1.5 m² plus two windows at 1.2 m² = 3.90 m².
3. Paintable wall: 43.20 − 3.90 = 39.30 m².
4. Two coats: 39.30 x 2 = 78.60 m² of coverage required.
5. At a conservative 9 m² per litre: 78.60 / 9 = 8.73 litres.
So the job needs two 5-litre tins, leaving 1.27 litres spare.
What the openings were worth
Ignoring the door and windows would have given 43.20 m² of wall and 9.60 litres. That is a 9% overstatement, and at these quantities it does not change what you buy — but on a job already sitting just under a tin boundary, 9% is exactly what tips it over. Most calculators skip this step.
What the coverage claim was worth
Run the identical room at an optimistic 13 m² per litre and the answer becomes 78.60 / 13 = 6.05 litres. That is one 5-litre tin plus a 2.5-litre, against two 5-litre tins.
The geometry did not move by a millimetre. The only thing that changed was which tin you believed, and it changed the shopping list. This is why the rate is an input on this page and why it defaults to the cautious end.
If you are unsure, round the coverage down rather than up. Running out two thirds of the way through a second coat means a second trip, a possibly different batch number, and a visible join.
Reference Data & Values
surface
metric coverage
imperial coverage
note
Primed, smooth drywall
11–13 m²/L
450–530 sq ft/gal
Best case; the figure most tins print
Previously painted, similar colour
9–11 m²/L
370–450 sq ft/gal
The realistic default for repainting
Bare plaster or new drywall
7–9 m²/L
290–370 sq ft/gal
Absorbs the first coat; prime first
Textured or rough render
6–8 m²/L
250–330 sq ft/gal
Behr quotes 250 at this end
Primer
5–7 m²/L
200–300 sq ft/gal
Covers less than topcoat; calculate separately
Frequently Asked Questions
Measure the perimeter, multiply by wall height, subtract doors and windows, multiply by the number of coats, then divide by the coverage rate on the tin. A 5 by 4 metre room with 2.4 metre walls, one door and two windows needs 8.73 litres for two coats at 9 m² per litre — two 5-litre tins.
Between 250 and 400 for one coat, depending on product and surface. Sherwin-Williams and Glidden quote 350 to 400 on primed smooth surfaces, Benjamin Moore 400, and Behr as little as 250 on porous, textured or previously unpainted work. Use the number on the tin you are actually buying.
Yes. On a typical room they are around 9% of the wall area — 3.90 m² out of 43.20 in the worked example above. That rarely changes the answer on a large job, but it is exactly what decides whether a borderline job needs one more tin.
Because they assume different surfaces. A UK tin claiming 13 m² per litre is claiming about 530 square feet per gallon, roughly 1.51 times the 350 quoted by an American tin. Neither is dishonest, but taking a figure from one market and applying it to the other will leave you a third short.
Two over an existing similar colour. Expect three when going dark to light, covering a strong colour, or painting bare plaster or new drywall, which drink the first coat. Volume scales exactly with coats, so a third coat adds precisely 50% to a two-coat estimate.
Primer is needed on bare plaster, new drywall, bare wood, or over a dramatic colour change. It covers noticeably less than topcoat — commonly 200 to 300 square feet per gallon, or 5 to 7 m² per litre — so calculate it separately with its own lower figure rather than reusing the topcoat number.
Down. Overestimating coverage means running out mid-coat, and a second trip risks a different batch number and a visible join where the two meet. Leftover paint is a minor waste; running out part way through a wall is a redo.
No. Those are normally a different finish, bought separately and brushed rather than rolled, so folding them into a wall estimate would distort both figures. Work them out separately, and note that trim paint typically comes in smaller tins because the areas involved are much smaller.
Measure each wall individually and add the areas, then enter the total. The perimeter-times-height shortcut only holds for a rectangular plan. Sloped ceilings, alcoves and bay windows all need measuring as separate rectangles and triangles.