A standard United Kingdom dense concrete block is 440 by 215 millimetres on face, and with a ten millimetre joint it occupies 450 by 225 millimetres of wall. That is 0.10125 square metres per block, so a square metre of wall takes 9.88 blocks. Merchants quote ten per square metre, which builds in a little over one per cent as a breakage allowance.
Block sizes are far less standardised than brick sizes. Thin-joint and aircrete blocks come in larger formats such as 620 by 215 or 440 by 215 at various thicknesses, and some are laid on a two or three millimetre glue joint rather than a ten millimetre mortar bed. Both dimensions and the joint are editable here for exactly that reason.
A wall 6 metres long and 2.4 metres high is 14.4 square metres, which needs about 143 blocks bare and 157 with a ten per cent allowance. Setting out matters as much as the count. That wall is 11 courses high and 14 blocks long, and if those numbers are not close to whole you will be cutting on every course. Adjusting the wall height by a few centimetres at the design stage is far cheaper than cutting a hundred blocks on site.
The formula
area- Face area of the wall in m²
length, height- Block face dimensions in mm, commonly 440 × 215
joint- Mortar joint in mm — 10 for a traditional bed, 3 for thin-joint
courses- Wall height divided by block height plus joint, rounded up
How it works, step by step
- Enter the length and height of the wall in metres.
- Confirm the block face dimensions from the delivery note or the pallet label.
- Set the joint thickness — 10 mm traditional, 2 to 3 mm for thin-joint systems.
- Read the block count on the gauge and the course setting-out below.
Worked examples
A 6 × 2.4 m inner leaf
14.4 m² at 9.88 blocks per m² is 143 blocks, or 157 with 10% waste — 3 pallets of 72. The wall is 11 courses of 14 blocks, which is very close to a whole number of courses.
Thin-joint changes the geometry
Laid on a 3 mm glue joint, each block occupies 0.09657 m², so you need 10.35 per m² rather than 9.88. Over 14.4 m² that is 7 more blocks, offset by using almost no mortar.
How to read your score
Frequently asked questions
Where does 10 blocks per square metre come from?
From 1 ÷ (0.450 × 0.225) = 9.88. The merchant rounds it to ten, which is convenient and quietly adds just over one per cent of spare. The derivation only holds for a 440 by 215 block on a 10 mm joint, so check your pallet label before trusting it.
Does block thickness change the count?
No. A 100 mm and a 140 mm block have the same face size, so the same number covers a square metre. Thickness changes the weight, the price, the mortar volume and the structural performance, but not how many blocks the face area takes.
Why does the course setting-out matter?
Because a wall that is not a whole number of courses high means cutting every block on the top course or adjusting the joint thickness across the whole wall. Blockwork is normally set out so that door and window heights land on a course, which is why 215 plus 10 millimetres is such a persistent dimension.
How heavy is a block?
A 100 mm dense concrete block weighs roughly 18 to 20 kilograms, a 140 mm one nearer 27, and aircrete blocks of the same size around a third of that. Manual handling limits are the reason aircrete dominates inner leaves, and the reason a pallet of dense blocks needs machine offloading.
Should I allow more than ten per cent?
Ten per cent suits a plain rectangular wall. Allow fifteen where there are many openings, since every reveal and every cill needs cut blocks, and dense concrete blocks do not cut cleanly.
Block wall reference
| Wall | Area | Blocks bare | With 10% waste | Pallets of 72 | Courses |
|---|---|---|---|---|---|
| 2 × 1 m | 2.0 m² | 20 | 22 | 1 | 5 |
| 3 × 2.4 m | 7.2 m² | 72 | 79 | 2 | 11 |
| 4 × 2.4 m | 9.6 m² | 95 | 105 | 2 | 11 |
| 6 × 2.4 m | 14.4 m² | 143 | 157 | 3 | 11 |
| 8 × 2.4 m | 19.2 m² | 190 | 209 | 3 | 11 |
| 10 × 2.7 m | 27.0 m² | 267 | 294 | 5 | 12 |
| 12 × 2.4 m | 28.8 m² | 285 | 313 | 5 | 11 |
| 15 × 3 m | 45.0 m² | 445 | 489 | 7 | 14 |
Computed at 9.88 blocks per m². Merchants normally quote a round 10 per m².