How many blocks and how much mortar for a wall

1. The net wall area

Multiply the length by the height of the wall, then deduct the openings (doors, windows). Example: a wall of 8.00 m × 2.80 m = 22.40 m², with a door of 0.90 × 2.10 m (1.89 m²) and a window of 1.20 × 1.00 m (1.20 m²): net area = 22.40 − 3.09 = 19.31 m².

2. Blocks per square metre

Divide 1 m² by the face of one block including the joint. With a 50 × 20 cm block and 1 cm joints: (0.51 × 0.21) = 0.1071 m², so about 9.3 blocks per m². With a 40 × 20 cm block: (0.41 × 0.21) = 0.0861 m², about 11.6 per m².

In the example with 50 × 20 blocks: 19.31 × 9.34 ≈ 180.3, rounded up to 181 blocks.

3. The waste margin

Add 5 % for a regular wall with few cuts, up to 10 % with many openings, corners or fragile products. Example: 181 × 1.05 = 190.05, so order 191 blocks — and always round up to full pallets when you buy by the pallet.

4. Mortar, cement and sand

Laying mortar is usually mixed at about 1 volume of cement for 4 volumes of sand, which corresponds to roughly 300 to 350 kg of cement per m³ of mortar. Count about 10 to 25 litres of mortar per m² of wall depending on the thickness of the blocks and of the joints.

Example at 20 L/m²: 19.31 × 0.020 = 0.39 m³ of mortar → about 0.39 × 350 = 135 kg of cement, i.e. 3 bags of 50 kg or 6 bags of 25 kg, plus about 0.45 m³ of sand.

• Check the manufacturer’s data: block dimensions, joint thickness and mortar consumption vary.
• Do not forget the concrete for footings, ring beams and lintels, and their reinforcement.

Reinforcement and finishing

Ring beams and lintels are reinforced concrete: the concrete calculator gives the volume and the dosage, and the rebar calculator the length and weight of the bars (weight in kg/m ≈ d² ÷ 162, with d in mm: a 10 mm bar weighs about 0.617 kg/m).

These estimates help prepare an order or a quote; they do not replace the design of a qualified engineer for load-bearing structures.