Block Wall Calculator

How block quantities are worked out

A standard 440×215mm concrete block covers 0.0945 m², so you need about 10 blocks per square metre. Allow around 0.3kg of mortar per block for thin-joint bedding, or more for traditional 10mm mortar beds.

Worked example: a 6 m garden wall at 2.4 m high

The wall face is 6 × 2.4 = 14.4 m². Standard UK 440 × 215 mm blocks lay at roughly 10 per square metre, so that is 144 blocks. Add 5% for cuts and breakages — about 8 more — and a couple of half blocks for starting each course at a corner. That gives 152 full blocks to order.

Foundations come first

A garden wall of this height normally sits on a concrete strip footing about 300 mm wide and 150 mm deep, poured below the topsoil (usually 500–600 mm down in Scotland to get under frost). Allow roughly 0.05 m³ of concrete per linear metre of 300 × 150 mm footing, so a 6 m wall needs about 0.3 m³ — typically 6–7 bulk bags of ballast plus cement, or one small ready-mix delivery.

Single skin or two skins?

A single 100 mm skin is fine for a low garden wall up to about 600 mm. Taller walls, walls in exposed positions, and anything load-bearing need either 215 mm solid blockwork or two 100 mm skins with wall ties at 450 mm horizontal and 300 mm vertical centres. Walls over about 1.8 m also need piers every 3–4 m — engineers specify these individually, so check before building.

Mortar, DPC and the details that get forgotten

Mortar quantities depend on joint width; for small jobs the 25 kg pre-mixed mortar bags quote a coverage figure, and building sand plus plasticiser is cheaper for anything over about 20 m². Fit a damp-proof course at the base of the wall where it meets ground level, and cap the top with a coping stone overhanging by about 25 mm so rain runs off instead of soaking the blockwork. Wall ties must slope slightly downwards towards the outer skin so any water runs back out.

Where block counts usually go wrong

Forgetting that the first course sets the bond and needs care; leaving openings (gates, doors) out of the area calculation; and buying lightweight blocks for a wall that will be drilled or bolted later — aerated blocks hold fixings poorly compared with dense concrete.