How to calculate wall area
Last reviewed 1 July 2026
Short answer
Gross area is length × height. Deduct each opening's width × height, multiplied by how many there are. For a gable, add the triangle: ½ × width × the height from the eaves to the ridge. Net area is what you multiply by the units per square metre.
Work it out for your project with the brick wall calculator →
The basic calculation
- Measure the length along the wall and the height from the top of the foundation, or from the DPC if that is where the brickwork starts.
- Multiply for the gross area.
- List every opening with its width, height and how many of that size there are.
- Total the opening areas and subtract from the gross.
- Multiply the net area by the units per square metre and by the number of skins.
net area = (length × height) − Σ(opening width × height × count)
Gables and sloping tops
A gable end is a rectangle plus a triangle. Measure the height to the eaves for the rectangle, then the additional height from eaves to ridge for the triangle.
Triangle area is ½ × base × height, where the base is the full wall width. A wall 6 m wide with 2.4 m to the eaves and a further 1.8 m to the ridge is (6 × 2.4) + (½ × 6 × 1.8) = 14.4 + 5.4 = 19.8 m².
For a wall with a single slope, such as a lean-to, average the two end heights and multiply by the length. That is exact for a straight slope, not an approximation.
When not to deduct
Standard estimating practice is not to deduct very small openings — typically anything under about 0.5 m². The reason is practical rather than arithmetic: a small opening generates cutting all around its perimeter, and the cuts consume roughly as much material as the opening saves.
For larger openings, always deduct. A patio door at 2.4 m × 2.1 m is 5 m², which is 300 bricks per skin — far too much to ignore.
If in doubt, deduct the opening and add a slightly higher waste allowance. That is more transparent than a rule about which holes count.
| Opening | Typical area | Deduct? |
|---|---|---|
| Air brick | 0.02 m² | No |
| Small vent or meter box | 0.1–0.3 m² | No |
| Small window | 0.5–1.0 m² | Yes, marginal |
| Standard window | 1.5–2.5 m² | Yes |
| Door | 1.8–2.0 m² | Yes |
| Patio door or garage opening | 4–7 m² | Yes, always |
Practical measuring
- Measure in metres to two decimal places. Rounding 2.37 m to 2.4 m across several walls compounds quickly.
- Check diagonals on anything you expect to be rectangular. Old walls rarely are.
- Measure openings as the structural opening, not the frame — the frame is smaller than the hole in the brickwork.
- Note the wall thickness while you are there. Whether it is single skin, solid or cavity doubles or halves the whole answer.
Safety notice: Measuring an existing wall is not the same as designing one
Safety notice: Material planning only