The formula
blocks per m² = 1 ÷ ((length + joint) × (height + joint))
Where it comes from
The standard concrete block measures 39 × 19 cm, and that is no accident: with a 1 cm joint the module comes to exactly 40 × 20 cm, which is 12.5 blocks per square metre and five courses per metre of height. The whole layout of blockwork is designed around those round numbers, which is why openings are best set out in multiples of 20 cm: every measurement that does not fit means cutting blocks, and cutting block is slow and looks bad.
How to work it out by hand
- Add the joint to both the length and the height of the block
- Multiply the two figures for the area of the module
- Divide 1 by that result: those are the blocks per m²
- Multiply by the wall area and add the waste allowance
What is worth knowing
Block takes considerably less mortar per square metre than brick, because there are fewer joints: twelve and a half pieces against thirty-two. In exchange it weighs more, and the wall needs plumbing with greater care, because an error in one course carries all the way up. If the wall is reinforced, hollow blocks are filled with concrete every so many pieces, and that concrete is not in this figure: it is worked out separately from the volume of the cavity.