Concrete Block Calculator
How many blocks a wall takes, from its area and the block size you are using.
Results
The dashed line marks the value you entered.
What this tool does
A block wall is counted by the space each block occupies **including its mortar joint**, not by the block on its own. That is the step people skip, and it is why hand counts come out short: a 440 × 215 block with a 10 mm joint really takes up 450 × 225, which works out at just under ten blocks a square metre rather than the ten and a half you get from the bare block. The default size here is the common 440 × 215 mm; type whatever yours measures, since block dimensions vary by country and by supplier. The waste allowance covers cuts at openings and the ones that arrive broken.
Formula
blocks = area ÷ ((length + joint) × (height + joint)) × (1 + waste)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
a | Area | m² |
bl | Block length | mm |
bh | Block height | mm |
j | Mortar joint thickness | mm |
wa | Waste allowance | % |
N | Blocks to buy | — |
PM | Blocks in each square metre of wall | — |
NW | Quantity | — |
Worked example
- Area20 m²
- Block length440 mm
- Block height215 mm
- Mortar joint thickness10 mm
- Waste allowance5 %
- Blocks to buy208
- Blocks in each square metre of wall9.88
- Quantity198
Limitations
- Always order a little more material than the calculated quantity. Cuts, breakage and pattern matching create waste that depends on the shape of the job.
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The calculation runs at full precision and only the display is rounded. If you copy an intermediate value and retype it, small differences can appear.