Tools
Paint calculator
Work out how much paint you need for the walls and ceiling of a room, taking off doors and windows and allowing for the number of coats and the type of surface.
How the calculation works
The calculator adds up the area of the four walls (twice the length plus twice the width, multiplied by the height), then deducts a standard door of about 1.9 m² (21 sq ft) and a standard window of about 1.5 m² (16 sq ft) for each one you enter. The remaining area is multiplied by the number of coats and divided by the paint's spreading rate.
By default we use 11 m² per litre (about 400 sq ft per US gallon), a typical figure for good-quality emulsion on smooth, previously painted walls. Check your tin: spreading rates range from roughly 8 to 16 m² per litre depending on the product. If you enter your own figure, the calculator uses it instead.
Finally, we add 10% for cutting in around edges, the paint left in the tray and roller, and touch-ups later. For new plaster or bare surfaces the spreading rate is reduced by 15%, and for textured surfaces by 20%, because they absorb more paint or have more surface area.
Tips for buying the right amount
- Two coats is the norm. One coat is rarely enough unless you are refreshing the same colour. Going from dark to light, or using a strong colour, may need a third coat or a tinted primer.
- Buy from the same batch. Tinted paint can vary slightly between batches. Buy everything you need at once and mix (box) the tins together before starting.
- Keep a little back. Store a small amount, labelled with the room and date, for touch-ups.
- Woodwork is separate. Skirting boards, doors and window frames usually need a different paint; about 1 litre of trim paint covers the woodwork of an average room in two coats.
For a full explanation with worked examples, read our guide How much paint do I need?, and to choose the sheen, see paint finishes explained.