How this calculator works
A standard UK concrete block is 440 × 215 mm on its face. With a 10 mm mortar joint each block takes up 450 × 225 mm of wall, which is about 9.9 blocks per square metre (merchants round it to 10). The mortar is the space between the blocks: the volume of the 450 × 225 mm envelope minus the block itself, times the block thickness.
Worked example: a 5 m long, 2 m high wall of 100 mm blocks is 10 m², so 99 blocks, or 104 with 5% for waste. The mortar comes to about 0.08 m³ wet (with 25% extra), which is 2 bags of cement and 0.14 tonnes of sand at 5 : 1.
Blocks for common wall sizes (5% waste)
Blocks of any thickness: the number does not change, only the mortar and the weight.
| Wall | Area | Blocks | Cement bags (100 mm) | Sand (100 mm) |
|---|---|---|---|---|
| 3 × 1 m | 3.0 m² | 32 | 1 | 0.04 tonnes |
| 5 × 1.2 m | 6.0 m² | 63 | 1 | 0.09 tonnes |
| 5 × 2 m | 10.0 m² | 104 | 2 | 0.14 tonnes |
| 8 × 2 m | 16.0 m² | 166 | 2 | 0.23 tonnes |
| 10 × 2.4 m | 24.0 m² | 249 | 3 | 0.34 tonnes |
Tips before you buy
- A wall needs a concrete foundation, whose depth and width depend on the height, the soil and the loads, and a damp-proof course above ground level. Ask your building control or a bricklayer.
- Dense concrete blocks are for below the damp-proof course and for external walls; lightweight blocks are for insulation and internal walls. Check the strength you need.
- Use a mortar to suit the block: weaker than the block, so that any movement shows in the joints, not the blocks.
- Wet very absorbent blocks lightly in hot weather, and cover fresh work in frost or rain.
- A garden wall over 1 m high needs piers or a thicker section, and one over 2 m (1 m beside a road) usually needs planning permission in England.
Tools & materials for this project
As an Amazon Associate we earn from qualifying purchases. These links do not change what you pay.
Frequently asked questions
How many concrete blocks per square metre?
About 10 for the standard 440 × 215 mm block (9.9 with a 10 mm joint), whatever the thickness of the block.
How many blocks do I need for a 5m x 2m wall?
99 blocks for the 10 m² wall, or 104 with 5% for waste.
How much mortar do I need for blockwork?
For 100 mm blocks, around 7 litres of wet mortar per square metre, or 8 with an allowance for spillage. For a 10 m² wall that is about 2 bags of cement and 0.14 tonnes of sand at 5 : 1, including some waste.
What size is a standard concrete block?
440 mm long, 215 mm high and 100, 140, 190 or 215 mm thick, so that two blocks plus a joint and one block plus a joint fit the brick coursing.
Add this calculator to your website
Running a blog, a contractor site or a class page? You can put this calculator on it for free. Copy the code below, and please leave the link underneath the calculator in place. The small script just resizes the frame to fit the results; if your site strips scripts, the calculator still works inside a scrolling frame.
Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.