Paint Calculator

How many litres of paint you need for your walls, allowing for doors, windows and coats.

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About this calculator

Paint tins tell you coverage per litre, but working backwards from your walls — minus doors and windows, times the number of coats — is where estimates go wrong. This calculator does that arithmetic and converts the answer into standard tin sizes.

The formula

Area = (Total wall length × Height) − Doors×1.9 − Windows×1.2 Litres = Area × Coats ÷ Coverage per litre

Worked example

A room with 14m of wall at 2.4m high, 1 door, 1 window, 2 coats at 12 m²/L:

Area = 33.6 − 1.9 − 1.2 = 30.5 m²
Paint = 30.5 × 2 ÷ 12 = 5.1 litres → two 2.5L tins won't quite do it; buy a 5L + small tin or three 2.5L

Common mistakes

Believing the optimistic coverage on the tin — textured or porous walls cover 20–30% less.
Skipping the second coat in the maths, then running out on a Sunday afternoon.
Painting over dark colours without budgeting an extra coat or a primer.

Frequently asked questions

Standard emulsion covers 10–14 m² per litre per coat on smooth walls. Use the lower end for fresh plaster (or mist-coat it first).

Yes — add the ceiling area (length × width of the room) to the wall length × height figure if painting it in the same colour, or run the calculator separately.

Round up to the next tin — leftover paint is invaluable for touch-ups, and batch colours vary slightly.

Assumptions & limitations

Standard deductions of 1.9 m² per door and 1.2 m² per window; skirting, frames and radiators not modelled.

Sources & verification

Manufacturer spreading-rate data — Coverage (m² per litre) is printed on the tin and technical data sheet, and differs by product and surface porosity — the default here is a typical emulsion figure

All sources last accessed 24 July 2026.

Formula last verified: 24 July 2026

Maintained by: Steve Davis, Founder & Editor — see our editorial standards

Spotted an error or an out-of-date figure? Report it — corrections are prioritised over all other work (see our corrections policy).

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