Concrete Block Calculator (CMU)
Works out concrete blocks or CMUs for a wall, with the mortar. Covers UK 440 × 215 blocks and US 8, 6 and 12 inch CMU.
Includes the waste allowance — this is the figure to order.
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The formula
units per m² = 1,000,000 ÷ ((face length + joint) × (face height + joint)) units = area × units per m² × (1 + waste)
Each unit effectively occupies its own face plus one joint along two edges, because the joint is shared with its neighbour. Working from the real face size and adding the joint is the only reliable way — nominal sizes already include the joint, so adding one again counts it twice.
What to watch out for
The arithmetic is simple; the choices around it are not:
- 10 blocks per square metre for the UK 440 × 215 with 10 mm joints. US 8 in CMU works out at 1.125 per square foot.
- Thickness does not change the count. A 140 mm block covers the same wall area as a 100 mm one — it just costs more, weighs more and uses more mortar.
- Blockwork uses less mortar than brickwork per square metre, because there are far fewer joints. Blocks are also usually shell-bedded rather than bedded solid.
Frequently Asked Questions
How many concrete blocks per square metre?
Ten, for the UK standard 440 × 215 mm block with 10 mm joints. A 6 × 2.4 m wall is 14.4 m², so about 144 blocks before waste.
How many CMU blocks per square foot?
1.125 for standard 8 × 8 × 16 in nominal CMU. A 100 square foot wall takes about 113 blocks.
How much mortar for concrete blocks?
Roughly 0.007 m³ per square metre of 100 mm blockwork at 10 mm joints — about a third of the 0.022 m³ the same area of brickwork takes, because there are a tenth as many joints.