📅 January 2026 • 7 min read

How to Calculate Paint for a Room

Step-by-step guide to measuring walls, accounting for doors and windows, choosing coverage rates, and getting the right amount of paint first time.

Measure Your Walls

The most accurate method is to calculate wall area directly:

  1. Measure the perimeter of the room (sum of all wall widths)
  2. Measure the ceiling height
  3. Wall area = Perimeter × Height (in metres or feet)

Example: 12 + 10 + 12 + 10 = 44 ft perimeter × 8 ft ceiling = 352 sq ft total wall area. In m²: 352 ÷ 10.764 = 32.7 m².

Doors and Windows

Subtract opening areas from your total:

  • Standard door: 1.85 m² (20 sq ft) — deduct per door
  • Standard window: 1.4 m² (15 sq ft) — deduct per window

Net area = Wall area − (Doors × 1.85) − (Windows × 1.4)

💡 Most decorators skip deductions for small rooms with only 1–2 openings — the extra paint serves as a useful touch-up buffer.

Coverage Rates

Paint coverage varies by quality. Manufacturer figures per coat:

  • Economy emulsion: ~10 m²/litre (300 sq ft/gallon)
  • Standard emulsion: ~11 m²/litre (350 sq ft/gallon)
  • Premium emulsion: ~12 m²/litre (400 sq ft/gallon)

Porous walls (new plaster, bare board) will give lower actual coverage. New plaster needs a diluted mist coat before any topcoat.

Number of Coats

  • 1 coat: Same colour maintenance repaint only
  • 2 coats: Standard for most rooms and colour changes
  • 3 coats: Dark-to-light colour changes or bare new plaster

Formula: Paint = (Net Area × Coats × 1.05) ÷ Coverage Rate

Calculating Cost

Multiply litres needed by your paint's price per litre. Regional 2026 prices:

  • UK standard: £12–£18/litre; premium: £20–£28/litre
  • US standard: $18–$28/gallon; premium: $38–$55/gallon
  • Canada: CA$25–$44/gallon; Australia: A$18–$34/litre

Pro Tips

  • Round up to the nearest litre/gallon — you cannot buy half a can
  • Buy the same batch number — batches can vary slightly in colour
  • Save at least one full can for touch-ups — especially custom-mixed colours

FAQ

Yes for: new bare plaster, walls with heavy sanding or patches, painting over oil-based paint, or covering very dark colours. Use a dedicated primer or a diluted mist coat on new plaster.
Standard emulsion: 2–4 hours. Vinyl silk: 4–6 hours. Never apply a second coat over a still-tacky first coat.