How it is calculated
Add up the length of all walls of the same thickness, multiply by the height and subtract the windows and doors. The area divided by the face of one block with its joints gives the number of blocks, plus a reserve for cutting. Blocks are sold by the cubic metre or the pallet, so the volume is shown too. Thin-joint adhesive is counted for the bed and head joints of every block at about 1.5 kg per m² per millimetre of joint; with joints of 8 mm or more the calculator counts ordinary mortar instead. The first row and every fourth row get bars in routed chases: two bars in walls 200 mm or thicker, one in thinner walls. The wall length, height and openings are remembered and filled in for the ICF calculator, and the wall length for the footing calculator too.
Blocks = (L × H − openings) ÷ ((l + joint) × (h + joint)) × (1 + reserve)Volume = blocks × l × h × tAdhesive, kg = blocks × (l + h) × t × joint × 1.5Reinforcement = reinforced rows × L × bars × 1.05Example
Walls 40 m long and 2.8 m high with 12 m² of windows and doors have 100 m² of masonry. With 625 × 250 mm blocks 300 mm thick and 3 mm joints you need 661 blocks (31 m³) including a 5 % reserve, about 781 kg of adhesive — 32 bags of 25 kg — and 252 m of 8 mm bar for 3 reinforced rows.
Tips from practice
- Lay the first row on a level bed of ordinary mortar over a damp-proof layer: any error here shows in every row above.
- Spread the adhesive with a notched trowel the width of the block so the joint stays 2–3 mm thick.
- Over windows and doors use lintels or U-blocks with a reinforced concrete beam, and put a ring beam under floor slabs and the roof.
- Before you lay the blocks, decide where the consumer unit, sockets and vent ducts go: chasing aerated concrete is easy, but vent openings are best left as you build.
Questions and answers
How many aerated blocks are in 1 m³?
It depends on the size: 21.3 blocks of 625 × 250 × 300 mm, 25.6 of 625 × 250 × 250 mm and 32 of 625 × 250 × 200 mm.
How much adhesive per m³ of aerated blocks?
With 2–3 mm joints about 15–30 kg per m³, usually one 25 kg bag per m³. The calculator counts it from the block size and joint thickness.
How thick should aerated block walls be?
Load-bearing external walls are usually 300–400 mm and partitions 100–150 mm. Thinner external walls need extra insulation; follow the design and local energy rules.
How many Thermalite blocks per m²?
About 10 blocks of 440 × 215 mm per m² with 10 mm joints (9.9 per m²), whatever the block thickness. Thin-joint blocks of 610 or 620 × 215 mm with 3 mm joints come to about 7.4 per m².
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