Paint Coverage Calculator
Work out how much paint a room needs from the wall perimeter and height, less doors and windows, for the number of coats — in litres or gallons and tins.
How many litres of paint the room takes.
How the paint coverage calculator works
Wall area is the perimeter of the room times the wall height, less the doors and windows. Multiply by the number of coats and divide by the paint's coverage — typically 12 m² per litre (about 350 sq ft per gallon) for an interior wall paint on a sealed surface; porous or dark surfaces cover less.
The tin count rounds up to whole 4-litre (or 1-gallon) containers. Ceilings and trim are separate: run the calculator again with the ceiling area as a single wall.
Formula: paint = (perimeter × height − openings) × coats ÷ coverage
Worked examples
| Inputs | Paint needed | Note |
|---|---|---|
| 14 m perimeter, 2.4 m walls, 1 door, 2 windows, 2 coats | 4.9 | 5.0 L — two 4 L tins |
| Same room, 3 coats | 7.4 | 7.4 L |
| 46 ft perimeter, 8 ft walls, 2 coats | 1.9 | about 1.9 gallons |
FAQFrequently asked questions
How much does a litre of paint cover?
About 12–16 m² per coat on a sealed, smooth wall; 8–10 m² on bare plaster or rough render. The tin states its own figure — use it in Detailed mode.
How many coats do I need?
Two for a colour change or new plaster (after a sealer); one may do for a repaint in the same colour. Dark-to-light changes can need three.
Should I subtract doors and windows?
Yes for large openings. Painters often ignore small windows because cutting in around them uses nearly as much paint as the glass would have.
What about the ceiling?
Run the calculator with the ceiling as one wall — for a 4 × 3 m room, a "perimeter" of 4 and a "height" of 3 — with no openings.
Where these figures come from
- The Engineering ToolBox — material densities, coverage and unit data — bulk densities, concrete at 2,400 kg/m³, board and sheet sizes
- Brick Development Association — brick sizes and quantities — 215 × 102.5 × 65 mm, 60 bricks per m²
- UK Government — Approved Documents (Building Regulations) — Building Regulations guidance
Last checked: September 2026. Coverage rates and material sizes are typical manufacturer figures — check the product you buy.