Blockwork quantities: blocks, mortar and the waste you actually incur
Block counts per square metre, opening deductions, mortar volume and a realistic waste allowance.
The engineering question
How many blocks and how much mortar does a wall need, and what should be added to the net quantity before ordering?
Worked example
The figures below were produced by running the Concrete Block Wall Calculator with the same inputs — during the build, from its authoritative engine.
| Blocks to order | 400 no |
|---|---|
| Bedding mortar | 0.8512 m³ |
| Mortar bags (approx.) | 29 no |
| Blocks before waste | 380 no |
| Assessment | 30.4 m² of blockwork → 400 standard blocks (incl. 5% waste) and ≈ 0.9 m³ of bedding mortar (≈ 29 mixed bags). |
- Net area is the gross wall less the openings.
- Standard blocks run about 12.5 per square metre including joints.
- Add a waste allowance that reflects the bond pattern and the number of corners.
Sanity check
Roughly 12–13 blocks per m² is the norm. A number far from it usually means the block size has not been matched to the module.
Tools for this problem
These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.
References & standards
- BS EN 771-1 — clay masonry units
- BS EN 771-3 — aggregate concrete masonry units
Related problems
Plaster and screed volume, cement bags and sand
Converting area and thickness into wet volume, dry volume and the bag count you will actually order.
Taking off concrete volumes without mis-ordering the pour
Where concrete volume actually goes — including the parts that are habitually forgotten — and how wastage and batching affect the order.
Also: Civil & Structural