Paint calculator

Wall area. The four walls of a rectangular room are two lengths and two widths, times the height. A room 4 m by 3.5 m with 2.5 m walls has 2 × (4 + 3.5) × 2.5 = 37.5 m² of wall.

Doors and windows. Take away the area you will not paint. Measure each one, height times width. A standard internal door is about 2 m², a typical window 1 to 2 m².

Litres. Area to paint times coats, divided by the coverage on the tin. This is the same formula as the Paint estimator in Construction Calc. The calculator starts from 10 m² per litre, a deliberately safe figure, so you do not run short. Many emulsions print a higher figure on the tin; enter yours for a closer answer.

Tins. The calculator picks the mix of 5 L, 2.5 L and 1 L tins that leaves the least paint over. Sizes and prices differ by brand, so check what your shop sells.

It depends on the paint and the surface, which is why every tin prints its own figure. At the 10 m² per litre this calculator starts from, 1 litre covers 10 m² of wall with one coat, or 5 m² with two. New plaster, rough walls and big colour changes soak up more, so expect less coverage there. For the full method, read how much paint per m² you need.

• The room is a rectangle. For an L shape, add up each wall's width and use that as the room's perimeter.
• Every coat covers the same area. A first coat on new plaster often needs a mist coat or primer.
• It does not include the ceiling, skirting boards or woodwork.