Block and Mortar Estimator

Estimating masonry for an extension comes down to a handful of per-square-metre figures. For a single-leaf 100mm wall you allow 10 blocks/m², and for the outer skin of a cavity wall you add 60 bricks/m². This estimator takes your wall area, subtracts the doors and windows, and turns those rates into a block count, a brick count, and the bags of cement and sand you need to bed them.

Enter your total wall area (or work it out from perimeter and height), deduct the openings, then pick your block type and whether the wall is single leaf or cavity. The result includes 10% wastage so the order sheet reflects what actually turns up on a pallet, not the theoretical minimum.

Where the per-m² figures come from

The block figure is not a guess. A standard UK block has a 440 × 215mm face, and once you add a 10mm bed and perpend joint the working module becomes 450 by 225mm. That gives just under ten blocks to the square metre, rounded up to 10 blocks/m² for ordering. Brickwork in stretcher bond works the same way and lands at 60 bricks/m².

Mortar scales with wall area too. A single-leaf wall uses roughly 0.5 bags/m² and 45 kg/m². A cavity wall has two leaves to bed, so the estimator doubles both. It also splits the sand: sharp sand for the mortar below the damp-proof course, where salt resistance matters, and building sand for the more workable mix above it.

Dense below the DPC, lightweight above

Most extension walls mix two block types. Dense concrete blocks go in for the first few courses below the DPC, where strength and moisture resistance count, and lightweight aerated blocks go above for their thermal performance and easier handling. The estimator lets you pick dense only, lightweight only, or a realistic mix of both, which changes which sand it assigns to each part of the wall. Your building control officer will check that the right block sits in the right place when they inspect the walls and blockwork.

A worked example

Take a single-storey rear extension with 40m² of wall face once you have taken off the bifold and window openings. At 10 blocks/m² the inner leaf needs the block count the estimator shows above, before you add 10% for cuts and breakage. Choose cavity construction and it layers 60 bricks/m² of facing brick on top for the outer skin.

For mortar on that same wall, the tool works from 0.5 bags/m² and 45 kg/m², doubling both because a cavity wall has two leaves to bed. Remember these are materials figures only. A £200 – £300 per day to lay them sits on top, and on a full extension that labour bill usually runs higher than the blocks and mortar combined.