How it is worked out
Wall area = 2 × (length + width) × height, minus about 21 sq ft per door and 15 sq ft per window. Paint = area × coats ÷ coverage per litre, with 10% extra.
Wallpaper needs about 15% extra for matching the pattern and trimming. Primer and putty are separate; a new wall usually needs one coat of primer.
Frequently asked questions
How much area does a litre of paint cover?
Interior emulsion covers about 100–140 sq ft per litre per coat on a smooth, primed wall. Textured or porous walls take more.