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Block calculator: concrete blocks and mortar for a wall
Count the concrete blocks for a wall and the mortar to lay them: 10 blocks per square metre for standard 440 × 215mm blocks with a 10mm joint, openings taken off as area, and the mortar split by designation. M4 (iii) is the normal mix for blockwork; aircrete inside the house can go down to M2.
What you need
Bricks and blocks
- Wall area
- 19.2 m²
- Dense concrete 7.3N, 100mm
- 202 blocks10 per m² plus wastageJewson block calculator; tradecalculator 9.88 per m²
- Pallets
- 3 × pallets of 72pallet size varies by block; 72 is typical for 100mm dense
Mortar
- Mortar (muck)
- 435 kg wet0.012m³ per m² of 100mm blockwork, about 2.2kg a blocktradecalculator block calculator; Tarmac mortar yields
- Cement (OPC), M4 at 1:1:6
- 3 × 25kg bagsparts are by volume, so this is a working figureMarshalls BTB4; MPA Mortar Data Sheet 19
- Hydrated lime
- 3 × 25kg bags
- Building sand (soft sand)
- 14 × 25 kg bags327kg. Bulk bags are about 800kg, not a tonneSelco; BMF bulk bag minimum 800kg
Round up to whole packs and bags. Figures include the wastage you chose and match TradeCalc in the app.
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Or claim your free profile link nowHow the maths works
- A standard UK block has a 440 × 215mm face. With a 10mm joint that is 9.88, call it 10, blocks per square metre (Jewson; tradecalculator). Dense, medium and aircrete blocks are the same size, so the count does not change with the type.
- Laid flat (on their side) a 100mm block makes a 215mm wall and you need twice as many: 1 ÷ (0.45 × 0.11) is 20 per m².
- Doors take off 1.9m² (about 19 blocks) and windows 1.4m² (about 14). The old rule of 12 and 8 blocks per opening is low.
- Mortar is worked out by volume per square metre of blockwork: 0.012m³ for 100mm, 0.016m³ for 140mm and 0.024m³ for 215mm (tradecalculator), at 1,800kg per m³ wet. That is about 2.2kg per 100mm block.
- The mix is a designation. M4 (iii), 1:1:6 or 1:5 to 6 with plasticiser, is right for inner-leaf blockwork and dense blocks below DPC. Lightweight and aircrete internal walls can use M2 (iv), 1:2:9. M12 (i) is only for engineering work (Marshalls BTB4 bulletin; MPA Mortar Data Sheet 19). Cement and lime come in 25kg bags; sand in bulk bags of about 800kg (Selco; BMF).
- Cavity walls need ties at 2.5 per m², which is 900mm horizontal by 450mm vertical centres, plus extra ties within 225mm of every opening (Ancon).
- Wastage at 5% covers cuts and breakages. Blocks come on pallets of roughly 72, so round up.
Worked example
An 8m × 2.4m wall in 100mm dense blocks is 19.2m². At 10 blocks per m² with 5% waste that is 202 blocks on 3 pallets, about 435kg of M4 mortar, 3 bags of cement, 3 bags of lime and 14 bags of building sand at 1:1:6.
Questions people ask
How many blocks are in a square metre?
10, for standard 440 × 215mm blocks with a 10mm mortar joint (the exact figure is 9.88). Laid flat it is 20. A door takes off about 19 blocks and a window about 14.
How much mortar does a block need?
About 0.012m³ per square metre of 100mm blockwork, which is roughly 2.2kg of wet mortar per block; 140mm blocks take about 2.9kg and 215mm blocks about 4.3kg. For 200 blocks of 100mm that is around 430kg of mortar: at M4 with lime, 3 bags of cement, 3 of lime and 320kg of sand.
What mortar mix for blockwork?
M4, designation (iii): 1:1:6 cement:lime:sand or 1:5 to 6 with plasticiser, for inner leaves and for dense blocks below DPC. Aircrete and lightweight blocks in internal walls can take a weaker M2 (1:2:9). A strong 3:1 mix is wrong for blockwork: the joint must be weaker than the block or the block cracks instead of the joint.
What is the difference between dense, medium and aircrete blocks?
Density, strength and weight. Dense 7.3N blocks go below ground and in load-bearing walls, medium density in most inner leaves, aircrete (Celcon, Thermalite) where insulation matters and for easy cutting. The count per square metre is the same. Thin-joint aircrete uses a 2 to 3mm adhesive bed instead of mortar, which this calculator does not cover.
How many wall ties do I need?
2.5 per square metre for a cavity wall, laid at 900mm horizontal and 450mm vertical centres in a staggered pattern, plus extra ties at 300mm vertical centres within 225mm of every opening, movement joint and roof verge (Ancon). Boxes are usually 250.
Do I need a foundation?
Any block wall over about 600mm high needs a proper concrete footing. Use the concrete calculator for the footing volume.
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