Paint quantity depends on three numbers: the area to cover, the number of coats, and the coverage of the product. The first two you measure; the third is printed on the tin, and it is the one people most often guess wrongly.

Step 1: area of the walls

Wall area = 2 × (length + width) × height

Deduct doors and windows. Standard openings make it quick: a 7 × 3 ft door is 21 sq ft; a 4 × 4 ft window is 16 sq ft. Add the ceiling (length × width) if it is being painted.

EXAMPLE

Room 12 × 10 ft, walls 10 ft high, one door, one window.
Walls = 2 × (12 + 10) × 10 = 440 sq ft
Less door (21) and window (16) = 403 sq ft

Step 2: litres

Litres = area × coats ÷ coverage per litre per coat (+ a small wastage allowance)

Coverage varies. It depends on the product, the surface texture, how absorbent the wall is, and whether it is primed. Use the figure printed on the tin for the surface you are painting, not a general rule.

EXAMPLE, CONTINUED

403 sq ft × 2 coats ÷ 100 sq ft per litre = 8.06 L, plus 5% = 8.5 L
Tins: 2 × 4 L + 1 × 1 L

Primer and putty

  • Primer: usually one coat. Same formula with the primer's coverage.
  • Wall putty: bags usually state coverage for two coats in sq ft per kg. Kg = area ÷ that coverage.

Buying tins

Round up to whole tins and choose the combination that leaves least over. Buy all tins of a colour at once, from the same batch where possible, so that shades match.

Tools

The paint area, paint, primer and putty calculators do these steps and suggest tin and bag sizes. Then send the customer a quotation.