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Concrete Block Calculator

Block counts come down to face area plus the mortar joint, which is why a 440 mm block does not go ten to the metre. Enter the wall size and block type below for the count, the mortar to lay them, and core-fill concrete if you are filling hollow CMU.

JointWall lengthHeight
Block wall: length, height and mortar joints
%

Blocks per square metre

Unit Face size Per m² Per 100 ft²
UK concrete block 440 × 215 mm 10 ~93
US CMU block 16 × 8 in 12.5 ~113
EU block 500 × 200 mm 9.3 ~87
UK brick 215 × 65 mm 60 ~560

All counts assume a 10 mm (3/8 in) joint. Add 5 per cent for cuts and breakages on a simple wall, 10 per cent where there are openings, returns or a lot of cutting — every cut block wastes most of a block.

Mortar quantities

Rule of thumb: 0.02 m³ of mortar per m² of blockwork, or 0.03 for brickwork, at a 10 mm joint. Bricks need more because there are six times as many joints in the same area. Get the cement and sand split from the sand and cement calculator, using 1:4 or 1:5.

Filling hollow blocks

A standard 8 × 8 × 16 in CMU takes about 0.3 ft³ (0.0085 m³) to grout solid, so 100 blocks needs roughly 1.1 cubic yards. Tick the core-fill option in the calculator and it adds the concrete volume and bag counts to the result. Use a wet, flowable mix for grouting — it has to travel down the cores without bridging.

Frequently asked questions

How many concrete blocks per square metre?

Ten standard UK blocks (440 × 215 mm) with 10 mm joints. Multiply wall length by height in metres, then add 5 per cent.

How much concrete to fill CMU cores?

About 0.3 ft³ per standard hollow block, so roughly 1.1 cubic yards per 100 blocks fully grouted.

How many bags of mortar per 100 blocks?

Five to six bags of 25 kg pre-mixed mortar, or about three bags of cement with 0.35 tonnes of building sand if mixing 1:5 yourself.

Do block walls need a concrete footing?

Yes. Typically twice the wall thickness wide and at least 300 mm deep for a garden wall, deeper on soft ground. Size it with the footing calculator.

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