Block calculator: how many blocks per m² and mortar bags

A 440 × 215 mm block laid with 10 mm joints covers 450 × 225 mm, so a square metre takes 9.88 blocks. An 18 m² wall needs 178 blocks; in 100 mm blocks one cubic metre of mortar lays about 1,500.

How many blocks per m²?

Standard UK blocks have a face of 440 × 215 mm. Laid with 10 mm joints, each one takes up 450 × 225 mm of wall, the coordinating size Blue Circle uses in its mortar figures. One square metre therefore takes 1 ÷ (0.45 × 0.225) = 9.88 blocks, whatever the thickness of the block.

A wall 10 m long and 2 m high with 2 m² of openings is 10 × 2 − 2 = 18 m², so it needs 178 blocks. The calculator always rounds up to a whole block: 10 m² takes 99.

100 mm blocks and mortar by wall area

Blocks of 440 × 215 mm laid with 10 mm joints: 1 ÷ (0.45 × 0.225) = 9.88 blocks per m². Weight from the Thermalite Shield brochure (7.0 kg per 100 mm block); mortar from the Blue Circle data sheet (1,500 blocks per m³, 75 bags of 20 kg per m³, 4 litres of water per bag). No breakage allowance.

Block weight

Weights depend on the block. The calculator uses the Thermalite Shield aircrete block from Forterra's brochure, a 600 kg/m³ block for external walls, foundations, partitions and separating walls, with a compressive strength of 3.6 N/mm². At the moisture content of a delivered block (23%), it weighs 5.2 kg in 75 mm, 7.0 kg in 100 mm, 9.8 kg in 140 mm, 10.5 kg in 150 mm and 14.0 kg in 200 mm. The 178 blocks of the example weigh 1,246 kg in 100 mm and 1,869 kg in 150 mm. Dense concrete blocks are much heavier: if you use them, check your supplier's weight.

Mortar: bags per wall

Blue Circle Quality Assured Mortar is a pre-mixed M4 (designation iii) 1:1:6 cement, lime and sand mortar in 20 kg bags. Its data sheet says one bag yields about 0.0135 m³, so one cubic metre is 75 bags, and that a cubic metre lays about 2,000 blocks of 75 mm, 1,500 of 100 mm or 1,000 of 150 mm in a single-leaf wall with 10 mm joints. Each bag takes 4 litres of water.

For the 18 m² wall in 100 mm blocks: 178 ÷ 1,500 = 0.119 m³ of mortar, so 9 bags and 36 litres of water. In 75 mm blocks it is 7 bags, in 150 mm 14 bags. The sheet gives no figure for 140 or 200 mm blocks, so the calculator shows none for those.

Openings, cuts and breakages

Deduct doors and windows by their area: a 1 × 1.2 m window takes off 1.2 m² and about 12 blocks. Cuts at corners and reveals, and the odd broken block, are not covered by any figure in the data sheets used, so no allowance is added automatically. If your merchant suggests one, enter it: with 5%, the 18 m² wall needs 187 blocks. The mortar stays the same, because breakage affects blocks, not joints.

Before you build

A block wall needs a sound, level foundation, usually a concrete strip footing worked out separately. For an extension or any wall that carries load, the block type, wall make-up and foundation are set by the approved design and building control, not by this calculator.

Common mistakes

• Dividing by the block face (440 × 215 mm) without the joint: you would get 10.57 blocks per m² instead of 9.88.
• Forgetting to deduct openings, or deducting them twice.
• Using an aircrete weight for dense concrete blocks.
• Ordering mortar for 100 mm blocks when building in 150 mm: it takes half as much again.
• Ordering the exact count with no spare for cuts.

Work out the footing with the concrete calculator, the floor with the screed calculator and any wall tiling with the tile calculator.