The paint calculator: how much paint a room actually needs

Paint is bought in cans and cans come in litres, but rooms come in whatever shape the builder left. This calculator does the one piece of maths every painter needs: the wall area, the coats, and what to actually buy. All browser-side - nothing you type leaves the page.

How the maths works

Wall area is the perimeter times the height: 2 x (length + width) x height. Each door is reckoned at about 1.9 square metres and each window at about 1.4, and they come off the total. A litre of typical emulsion covers roughly 10 square metres per coat - that is the industry's usual figure, and it varies by brand, so the can's own number wins ties. The calculator adds 10 per cent for trays, roller skins and the bit that drips, then rounds up to the nearest half litre.

The adjustments that actually matter

Dark over light, or any big colour change: add a coat - the calculator's three-option input is where honesty lives; two coats is the usual answer, three is common over reds and navies. Bare new plaster: it drinks - a diluted first coat (or a primer) before the counting even starts. Textured or rough walls: up to 20 per cent more paint; the surface area is bigger than it looks. The ceiling: not in this sum - it adds roughly the floor's area per coat, which for most rooms is one extra litre or so.

Fix the wall first, always

No calculator saves you from painting over a problem. Damp shows through every coat - diagnose it first (which damp is it?) and never seal it in with paint (painting over damp, explained). Humid rooms need paint chosen for them (humidity and paint), and prep is the difference between one repaint and three (the mistakes list). The cost side of the job - paint, kit, your time - sits with repair costs, estimated.

Approximate by design: the calculator is a planning tool, not a quotation. Buy the second can returnable if your shop allows it, and note the colour and brand when a room comes out right - future you will need exactly that line.