Estimate how much paint you need for a room before you buy. Add wall size, ceiling choice, doors and windows, coats, coverage per litre and your preferred tin size to work out litres needed, tins to buy and likely spare paint.
Most UK paint brands quote about 10-12 m² per litre for smooth surfaces, but rough plaster, fresh plaster, bold colour changes and textured walls can increase usage. This tool adds coats and wastage so you can buy a safer amount.
This calculator multiplies room perimeter by wall height, optionally adds the ceiling, subtracts openings and then applies coats and wastage before converting area into litres.
For a rectangular room, total wall area is 2 × (length + width) × height. If you are also painting the ceiling, the calculator adds length × width. It then subtracts the area taken up by doors and windows, multiplies by the number of coats, adds any wastage allowance and divides by the coverage rate per litre.
Common tin sizes include 1L, 2.5L, 5L and 10L. Buying one larger tin is often cheaper per litre, but smaller tins can reduce waste if you are decorating a small room or using multiple colours.
Use this as a planning tool before ordering paint. Coverage figures vary by brand and surface prep, so always check the label on the specific product you buy.