Paint Quantity Calculator

Enter your wall area, ceiling area, and length of woodwork, and this calculator tells you how many litres of paint to buy and roughly what it costs. It works from the standard planning coverage of 12 m²/L, so the litre figures line up with how decorators actually estimate a job rather than the optimistic number printed on the tin.

Paint is one of the easiest things to under-order on an extension. You measure the walls, buy what the tin claims it covers, and run out halfway through the second coat. This tool sizes the order properly and rounds up to whole tins, because you cannot buy a part-tin and a small buffer for touch-ups is always worth having.

How coverage works

The quantity comes from three inputs: the area, the number of coats, and how far the paint stretches. Multiply the area by the coats, divide by the coverage rate, and you have your litres. The calculator uses 12 m²/L for coloured matt emulsion, which is a conservative real-world figure. Tin labels often quote a higher number, but that assumes a pale colour on a sealed, perfectly smooth surface. Deep colours carry more pigment and cover less, and a freshly skimmed wall drinks the first coats, so planning to the label leaves you short.

Woodwork is handled separately. Skirting boards, architraves, and door frames are estimated from the running length you enter, since a satinwood or gloss tin covers differently from a wall emulsion and you buy it as its own product.

New plaster changes everything

If your extension has fresh plaster, the walls are not ready for full-strength paint. They need a mist coat first: emulsion thinned at 10 to 40% water by volume (70:30 default), brushed or rolled on to seal the porous surface and give the top coats something to bond to. Skip it, or use a vinyl emulsion, and the finish lifts off in sheets within months. The plaster is usually ready after 5-7 days, once it has dried to a uniform pale pink.

The mist coat is not counted in the litre totals here, because it is heavily watered down and one tin of contract matt goes a long way. Order it as a separate line and size it from the tin.

A worked example

Say you have a room with roughly 60 m² of wall to paint over two coats, plus a ceiling and a run of skirting. Multiply the wall area by the two coats, divide by the 12 m²/L coverage figure, and the calculator returns the litres you need for the walls, then does the same for the ceiling and woodwork. It rounds each up to the nearest sensible tin combination and adds a supply-cost range on top.

For a whole room of walls and ceiling in good trade emulsion, two coats, the paint bill lands around £40 – £80. That is materials only. It does not include the mist coat, primer for bare woodwork, or the decorator's labour, all of which sit alongside the numbers this tool gives you. Use the result to buy the right amount in one trip instead of making three runs to the merchant.