The formula
bricks per m² = 1 ÷ ((length + joint) × (height + joint))
Where it comes from
The calculation does not use the size of the brick but the size of the module: the brick plus the mortar joint around it. A perforated brick of 24 × 11.5 cm with a 1 cm joint really occupies 25 × 12.5 cm, which gives 32 bricks per square metre instead of the 36 you get by ignoring the joint.
How to work it out by hand
- Add the joint to both dimensions of the brick
- Multiply to get the area of the module
- Divide 1 by that result: those are the bricks per m²
- Multiply by the wall area and add an allowance for breakages
What is worth knowing
An 8 % allowance covers breakages in transit and the cuts at corners and returns. Every square metre of brickwork takes about 0.03 m³ of mortar, which is around 10 kg of cement. If the wall is to be left exposed, raise the allowance: more bricks get rejected on looks than on a wall that will be rendered.