Paint calculator

How much paint do I need?

Paint needed = area to paint × number of coats ÷ coverage per coat. Work out the wall area as the room's perimeter × wall height, take off doors and windows, and add the ceiling if you're painting it.

Walls          = 2 × (length + width) × height
Area to paint  = walls − doors − windows (+ ceiling: length × width)
Paint          = area × coats ÷ coverage per coat × (1 + extra %)

1. Pick Region presets and Units at the top. They set the coverage, the door and window sizes, and the tin sizes, which you can all change.
2. Enter each room's length, width and wall height, with the number of doors and windows. Tick Paint the ceiling too if you are.
3. Or switch to Wall area if you already know the area, for example from the plaster calculator.
4. Set the coats and the coverage from your tin. Add primer for new plaster or bare surfaces.
5. Open Openings, wastage and tins to change door and window sizes, the extra allowance and the tin sizes on sale.

Worked example: a 4 × 3 m room with 2.7 m walls

The walls are 2 × (4 + 3) × 2.7 = 37.8 m². Less one door and one window that is 34.8 m². Two coats at 12 m² per litre, plus 10%, need 6.38 litres: 1 × 5 L + 2 × 1 L.

1. Walls: 2 × (4 + 3) × 2.7 = 37.8 m².
2. Less a door (1.6 m²) and a window (1.4 m²): 37.8 − 3 = 34.8 m².
3. Paint: 34.8 × 2 coats ÷ 12 m²/L × 1.10 = 6.38 L.
4. Tins: 1 × 5 L + 2 × 1 L = 7 L: the fewest 1, 2.5, 5 and 10 L tins that leave less than one 1 L tin over.
5. With the ceiling (4 × 3 = 12 m²) the area is 46.8 m² and the paint 8.58 L.

How much paint for a 10 × 10 or 12 × 12 room

Walls only. To add the ceiling, tick Paint the ceiling too in the calculator above.

Paint coverage per litre and per gallon

Coverage depends on the paint and the surface, so use the figure on your tin. The calculator starts from 12 m² per litre per coat, 350 sq ft per US gallon, or 129.2 sq ft per litre per coat in Pakistan and India.

Where these come from: Dulux Trade Vinyl Matt gives up to 17 m² per litre, an "up to" figure, so the calculator starts lower. Behr quotes 250–400 sq ft per gallon for its Premium Plus interior paint, and the US preset sits inside that range. Asian Paints Tractor Emulsion is quoted for two coats: 10–12 m² per litre on its product sheet and 130–150 sq ft per litre on the Indian product page. That is the area one litre covers with both coats on, so it goes about twice as far per coat; the sheet gives 18–22 m² per litre for one coat. Berger Pakistan lists 12–16 m² per litre per coat for its emulsions. The Pakistan and India presets start at the low end of Berger’s range, 12 m² (about 129 sq ft) per litre per coat, so you’re unlikely to run short.

New plaster soaks up more paint: Asian Paints specifies three coats on a fresh surface and two for repainting. Behr gives its range as depending on the application method and how porous the surface is, so rough or porous walls and spraying use more.

Litres vs gallons

A US gallon is 3.785 litres and an imperial (UK) gallon is 4.546 litres. The calculator's United States preset uses quart, gallon and 5-gallon cans; the other presets use 1, 2.5, 5 and 10 L tins, which you can change.

Both gallons have exact definitions: 3.785411784 L for the US gallon and 4.54609 L for the imperial gallon (UK Weights and Measures Act, NIST). With the United States preset the calculator works in US gallons and cans; elsewhere it works in litres, and you can type the tin sizes your shop sells.

Choosing tins

The calculator picks the fewest tins that hold at least the paint you need without leaving a whole smallest tin or more unused; between equal numbers of tins, it picks the one with less left over. Change the tin sizes to match what your shop sells.

For the 6.38 litres in the worked example, from 1, 2.5, 5, 10 L tins, it suggests 1 × 5 L + 2 × 1 L: 3 tins holding 7 L, 0.62 L over. Keep a little of each colour for touch-ups.

Primer, putty and what's not included

Prime new plaster, bare wood and repairs before the top coats. The calculator adds one primer coat at the coverage you enter. Wall putty, fillers and trim paint are not included.

We found no primary source for primer coverage, so the calculator starts at 10 m² per litre and asks you to check the tin. Wall putty, where it's used, is sold by the kilogram and its use depends on how uneven the wall is, so estimate it from the putty maker's figure. Paint for skirting, door frames and window frames isn't included; work it out separately.

Still plastering? Work out cement and sand first with the plaster calculator.

Sources

Constants and limits come from these standards, manufacturers' data and trade bodies. Where practice varies, the calculator lets you change the value.

• Dulux Trade Vinyl Matt product data sheet: spreading rateAkzoNobel (Dulux Trade) · current
• Tractor Emulsion product information sheet: coverageAsian Paints · current
• Weights and Measures Act 1985, Schedule 1 (gallon = 4.54609 litres)UK legislation · 1985
• NIST Guide to the SI, Appendix B.9: conversion factorsNational Institute of Standards and Technology · SP 811

Found a value that doesn’t match your local practice? Tell us and we’ll review it.