Block Calculator UK
This block calculator uses the UK rule of 10 blocks per m² for standard 440 × 215 mm concrete blocks with 10 mm joints. A wall 5 m long and 2 m high is 10 m², so it needs 100 blocks, or 110 with a 10% allowance for cuts and breakages, plus about 2 bags of cement and 170 kg of sand.
- Standard block440 × 215 mm face
- Blocks per m²10
- Waste allowanceat least 10%
Updated November 2026 · Works in metric units
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How many blocks per m²?
A standard UK concrete block has a face of 440 × 215 mm. With a 10 mm mortar joint each block covers 450 × 225 mm of wall, which is 0.10125 m², so you need about 9.9 blocks per m². Builders round this to 10 blocks per m². The thickness (100, 140 or 215 mm) does not change the count per m² because the face size is the same.
| Wall area | Blocks (10 per m²) | With 10% waste |
|---|---|---|
| 5 m² | 50 | 55 |
| 10 m² | 100 | 110 |
| 20 m² | 200 | 220 |
| 50 m² | 500 | 550 |
How to calculate blocks for a wall
- Multiply the wall length by its height to get the area in m².
- Subtract any door and window openings.
- Multiply by 10 for a single leaf, or by 20 for two leaves of blockwork.
- Add at least 10% for cuts, breakages and spares.
Example: 5 m × 2 m = 10 m². 10 × 10 = 100 blocks, plus 10% gives 110. For a brick-and-block cavity wall, count the blocks for the inner leaf here and the bricks for the outer leaf with our brick calculator (60 bricks per m²).
Mortar, sand and cement for blockwork
Blockwork uses less mortar per m² than brickwork because there are fewer joints. A 100 mm wall has about 0.0067 m³ of joint per m², or roughly 2 kg of mixed mortar per block once you allow for waste. A 4:1 mix suits most walls; use 3:1 below the damp-proof course and a weaker 5:1 or 6:1 mix for lightweight aerated blocks if the manufacturer recommends it.
| Blocks (100 mm) | Mortar | Cement (4:1) | Sand (4:1) |
|---|---|---|---|
| 100 | about 210 kg | 2 bags | about 170 kg |
| 250 | about 520 kg | 5 bags | about 420 kg |
| 500 | about 1,050 kg | 9 bags | about 840 kg |
Choosing concrete blocks
- Dense blocks are strong and heavy, used for foundations, load-bearing walls and below ground.
- Lightweight and aerated blocks are easier to handle, cut and insulate better, and are common for inner leaves and internal walls.
- Trench blocks (usually 300 mm or wider) are used in foundations instead of two leaves of blockwork.
- Buy from one batch and check the compressive strength your design or building control requires.
Frequently asked questions
How many blocks are in 1 m²?
About 10 standard 440 × 215 mm blocks per m² with 10 mm joints, before any allowance for waste.
How many blocks do I need for a 10 m² wall?
100 blocks for a single leaf, or 110 with a 10% waste allowance.
How many blocks per m² for a 140 mm wall?
Still 10 per m². Thickness changes the weight and mortar needed, not the number of blocks per m² of wall face.
How much cement do I need for 100 blocks?
For 100 mm blocks in a 4:1 mix, allow about 2 × 25 kg bags of cement and around 170 kg of building sand.
How many blocks are in a course?
One block per 450 mm of wall length, and each course adds 225 mm of height, so a 2 m high wall has about 9 courses.
Sources and method
We use official UK guidance and recognised formulas, and test each calculator against worked examples. Results are estimates to help you plan, not professional advice. See how we work.