Paint Calculator
Work out how much paint you need for a room — given wall dimensions, doors, windows, and the number of coats.
With 38.55 m² of paintable wall and 2 coats, you'll need 7.01 L of paint.
Paint to buy
Results update as you type. Coverage assumes a typical emulsion at 11 m²/L; check the tin/can label — porous surfaces, dark-to-light colour changes, and primer coats all need more paint.
Formula
Wall area is perimeter × height:
2 × (length + width) × height.
Subtract the typical area for each door and window (1.85 m² and 1.4 m² respectively),
multiply by coats, and divide by coverage (~11 m²/L for emulsion, ~350 sq ft/gal for US).
The calculator rounds up to whole tins because partial cans don't sell.
When this calculator helps
Standing in the paint aisle guessing how many tins to grab is how most people either run out halfway up a wall or end up with three spare litres they will never use. This calculator takes the guesswork out: feed in your room's length, width and height in metres, tell it how many doors and windows to subtract, choose your number of coats, and it works out the litres of emulsion you actually need and how many 2.5 L or 5 L tins that comes to.
It is built for the everyday UK decorating job — repainting a lounge before guests arrive, freshening a rented flat back to magnolia before handing back the keys, or pricing up a feature wall in a deep colour. Because you set the coats and the calculator uses a conservative coverage figure, the litres it returns are a realistic shopping target rather than the optimistic number on the side of the tin.
How to read your result
The result has two figures that matter. The paintable area in square metres is the wall surface left once doors and windows are deducted, and it is worth a sanity check — a typical 4 m by 5 m room at 2.4 m high gives roughly 40 m² of wall before openings. The litres figure is that area multiplied by your number of coats, then divided by the coverage rate of about 11 m² per litre.
From the litres, the calculator rounds up to whole tins at standard UK pack sizes. Always round up rather than down: paint is sold in fixed 2.5 L and 5 L tins, and a job that needs 6 L means buying 7.5 L, not hoping 5 L stretches. The leftover is not waste — sealed, it keeps for a couple of years and covers the inevitable scuffs and touch-ups.
A worked example
Take a 4 m by 5 m bedroom with a 2.4 m ceiling. The four walls measure (4 + 5) × 2 × 2.4 = 43.2 m². Subtract one door at about 1.85 m² and two windows at roughly 1.4 m² each, and you are left with around 38.5 m² of paintable wall. For two coats that is 77 m² of coverage, which at 11 m² per litre needs about 7 litres. The calculator rounds that up to one 5 L tin plus one 2.5 L tin — 7.5 L in total — leaving a small reserve for touch-ups.
Common mistakes to avoid
Most paint shortfalls come from a handful of avoidable errors when estimating.
- Forgetting the second coat — a single coat almost never covers evenly, especially over a colour change, so set coats to two unless you are repainting like-for-like.
- Not deducting large openings — French doors or a full-height window remove far more wall than the standard averages, so adjust or round up an extra tin.
- Skipping primer on bare or dark walls — fresh plaster and deep colours drink paint, and without an undercoat you may need three finish coats instead of two.
- Trusting the manufacturer's coverage on textured or porous walls — woodchip, freshly plastered or chalky surfaces can use 15–20% more than a smooth primed wall.
UK paint and finish notes
British walls are usually finished in water-based emulsion (matt, eggshell or silk), with kitchens and bathrooms often taking a moisture-resistant or wipeable grade. Coverage figures on UK tins assume a smooth, primed surface, so older homes with Artex, woodchip or lime plaster will need more than the label suggests. If you are painting fresh plaster, a mist coat — emulsion thinned with water — soaks in as a primer before your finish coats go on, and that mist coat counts as an extra layer when you are buying.
Tin sizes here are standardised at 2.5 L and 5 L for most emulsions, with 1 L and 10 L turning up for trade and specialist colours. The 5 L tins are usually the better value per litre, but only if the job is big enough to justify them before the opened paint starts to skin over. For a single small room, two 2.5 L tins often waste less than one part-used 5 L.
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Frequently asked questions
How much paint per square metre?▾
Manufacturer coverage figures for emulsion run 11–13 m²/litre per coat on a smooth, primed surface. We default to 11 m²/L because real-world coverage usually runs lower than the marketing figure — porous walls, fresh plaster, deep colours, and DIY brushwork all reduce coverage. Always overestimate slightly: running out mid-room is more annoying than buying an extra tin.
How many coats do I need?▾
Two for most repaints. Three (or two over a primer/undercoat) for: dark colour over light, light colour over dark, fresh plaster, or stained walls. One coat is only realistic for the same colour as before with a high-opacity 'one-coat' product, and even then it usually betrays brush marks.
Why does the calculator subtract doors and windows?▾
Because you don't paint them with wall paint. We use typical UK averages (door ~1.85 m², window ~1.4 m²) to deduct from the paintable area. If your doors or windows are unusually large (French doors, full-height glazing), the calculator will under-deduct slightly — round up your tin count by one as insurance.
Should I add extra for waste?▾
Already built in by the conservative coverage figure (11 vs the manufacturer's 12–13). For unusual jobs — textured walls, fresh plaster, dark accent walls — add another 10–15% to your purchase. Paint stores well sealed for ~2 years; modest leftover stock is useful for touch-ups.
Are 5 L tins better value than 2.5 L?▾
Almost always — 5 L tins typically save 20–30% per litre vs 2.5 L. The catch is that opened paint deteriorates faster than sealed; if your job needs only 3 L, two 2.5 L tins waste less than one 5 L. Check brand price-per-litre before assuming.
Is ceiling paint different from wall paint?▾
Yes — ceiling-specific paint is matte, low-spatter, and often non-drip. It's possible to use wall emulsion on a ceiling (just thin it slightly) but ceiling paint generally produces a better finish and is no more expensive. This calculator only handles wall area; for ceilings, calculate length × width separately.