How many blocks per m²?
Standard UK concrete blocks have a face size of 440 × 215mm. With 10mm joints, each block covers 450 × 225mm, or 0.101m², so you need 10 blocks per m². The thickness (100mm, 140mm or 215mm) changes the strength, weight and amount of mortar, not the number of blocks.
Common UK block types
| Block | Approx. weight | Typical use |
|---|---|---|
| 100mm dense | 18–20 kg | Inner/outer leaves, garden walls, partitions |
| 140mm dense | 26–28 kg | Separating walls, load-bearing walls |
| 215mm solid dense | ~40 kg | Foundations below ground, retaining walls (laid flat) |
| 100mm aerated | ~7 kg | Inner leaf of cavity walls, partitions |
Blockwork tips
- Never lay blocks over about 1.5m high in one day: mortar needs time to stiffen.
- Bond blocks half over half for strength and tie into existing walls with wall starter kits.
- Use frost-resistant dense blocks below ground level; aircrete blocks are suitable below ground only where the manufacturer allows.
- Keep blocks covered and dry: wet blocks shrink as they dry, which can crack plaster.
Worked example: a 5m × 2.4m wall in 100mm dense block
12m² of wall × 10 blocks per m² = 120 blocks, plus 5% for cuts and breakages = 126 blocks. Each 100mm block takes about 2kg of dry mortar, so 252kg in total. In a 4:1 mix that's about 50kg of cement and 200kg of building sand: 3 × 25kg bags of cement (just over two) and 9 small bags or a part bulk bag of sand. The blocks alone weigh nearly 2.5 tonnes, so arrange a kerbside crane delivery close to where you're building rather than carrying them through the house.
The same wall in 100mm aircrete weighs under a tonne. That's why aerated blocks are popular for inner leaves and partitions, where they can be cut with a saw and carried two at a time.
Cutting and laying blocks
- Dense blocks: score all round with a bolster and club hammer and snap them, or use a block splitter or a disc cutter with dust suppression.
- Aircrete blocks: cut cleanly with a coarse masonry handsaw.
- Bond: lay half over half, starting each course from the corners with a string line between them. Check level and plumb every couple of courses.
- Joints: keep them a consistent 10mm, so 10 blocks per m² holds true and courses line up with door and window heights.
Regulations and structure
New external walls, extensions and any load-bearing blockwork come under the Building Regulations and need building control approval. The block type, strength, wall ties and foundations will be set by the design, and that always overrides general guidance like ours. Garden walls and non-structural partitions are simpler, but still need a proper footing. The foundation calculator covers strip footings and trench blocks.
Common mistakes
- Laying too high in one day. Wet mortar squeezes out and the wall can lean or slump. Stop at about 1.5m and carry on the next day.
- Using aircrete where it isn't rated. Some aerated blocks can't go below ground or carry heavy point loads. Check the maker's data sheet.
- No movement joints on long walls. Long runs of blockwork, especially lightweight block, need movement joints at intervals set by the designer or block maker.
Facing the wall in brick? The brick calculator handles cavity walls with a block inner leaf. For a rendered finish, use the render calculator. For mortar on its own, use the mortar calculator.