Block calculator blocks and mortar

The block count, step by step

A standard UK block has a 440 × 215mm face. Add a 10mm joint to the end and the bed and each block covers 450 × 225mm of wall, which is 0.10125m². So 1m² takes 9.88 blocks. The calculator multiplies the net area by that, adds your wastage and rounds up.

The mortar comes from the number of blocks laid and their thickness, using a mortar maker's published yields for blockwork. That volume becomes building sand in tonnes and cement in whole bags, using the same method as the brick calculator.

Worked example: the inner leaf of a single-storey extension, 7.2m long and 2.5m high, with one door opening 0.9m wide and 2.1m high. 100mm blocks, 5% wastage, 10% extra mortar and a 1:4 mix.

• Gross area: 7.2 × 2.5 = 18.0m².
• Less the door: 0.9 × 2.1 = 1.89m². Net 16.11m².
• Blocks laid: 16.11 × 9.877 = 159.1.
• Plus 5%: 159.1 × 1.05 = 167.1, so 168 blocks. In packs of 120, that is two packs with 72 to spare.
• Mortar: 159.1 ÷ 1,500 blocks per m³ = 0.106m³. Plus 10% = 0.117m³.
• Sand and cement: 0.117 × 1,820kg = 212kg dry. A 1:4 mix gives 42.5kg of cement, so 2 bags. Building sand is the rest: 0.17 tonnes.
• In 140mm blocks the count is the same, but the mortar rises to 0.163m³: 3 bags of cement and 0.24 tonnes of sand.

How many blocks in a square metre

9.88 blocks per m² of wall, laid on edge with 10mm joints. That is the figure in H+H's Products and Applications Guide for its 440 × 215mm Celcon blocks, and it is the same for every thickness. The block's thickness changes the mortar and the weight, not the count.

Most people round it to 10. On 100m² of blockwork that rounding adds 12 blocks, which your wastage covers anyway. The 440 × 215mm face is the same across the big makers: Thermalite's Shield Block data sheet and Tarmac's mortar yields are both based on it.

Laid flat, a block shows its 440 × 100mm side, so the wall is 215mm thick and each block covers 450 × 110mm with joints. That is 20.2 blocks per m², about twice as many. Larger formats cut the count: H+H says its 630 × 250mm Jumbo Bloks need as few as 6.3 per m², laid with thin joint mortar.

Mortar for blockwork

Allow about 0.0066m³ of mortar for each m² of 100mm blockwork, and more for thicker blocks. Tarmac's Blue Circle Quality Assured Mortar data sheet gives how many blocks a cubic metre of mortar lays in a single leaf wall with 10mm joints: 2,000 blocks at 75mm thick, 1,500 at 100mm and 1,000 at 150mm. Those three figures fall on a straight line, 150,000 divided by the thickness in mm. The calculator uses that line, which gives about 1,071 blocks per m³ for 140mm and 698 for 215mm.

By our arithmetic, Tarmac's yields are the volume of the joints themselves, with nothing allowed for mortar dropped on the scaffold or left in the board at the end of the day. So the calculator adds 10% to the mortar by default. That is our allowance, not Tarmac's. Change it to suit how you work.

To turn mortar into sand and cement, it allows 1,820kg of dry materials for each m³, from the yield of a 20kg bag of Hanson's ready-mixed mortar, and splits the weight by your mix. The Mineral Products Association's table lists cement to sand at 1:3 to 4 for an M6 mortar, with 1:5 to 6 giving the weaker M4. Both ratios are by volume. Use what the drawings or the engineer specify.

These figures are for ordinary 10mm joints. Aircrete blocks laid on thin layer mortar use far less, because the joint is much thinner. Follow the block maker's figures for those.

Dense, lightweight or aircrete

The count is the same whichever block you use. The weight is not, and on a long day of lifting that matters as much as the price.

Aircrete is the lightest. H+H lists its Standard Grade Celcon block at 7kg in 100mm, 10kg in 140mm and 16kg in 215mm, at typical moisture when laid. Thermalite gives 7.6kg for a 100mm Shield Block. A dense aggregate block is far heavier: Forterra lists its Conbloc Evalast Block 1990 at 19.4kg in 100mm and 27.2kg in 140mm, at 3% moisture.

H+H prints in bold the weights that go over the recommended limit for one person lifting repeatedly. Plan the job round that, with two-person lifts or lighter blocks.

Which block a wall needs is a design question. Forterra's data sheets point you to Approved Document A and the project's structural engineer for strength and thickness. They also say that below the damp proof course the block must suit the site's soil conditions, and that blocks in an external leaf need a suitable finish.

Openings, packs and what is not counted

Take off every opening at its structural size, the hole the frame sits in. A standard door opening of 0.9 × 2.1m is 1.89m², or about 19 blocks.

Blocks come in packs, and the count depends on the thickness. H+H's packs of 440 × 215mm Celcon hold 160 blocks at 75mm, 120 at 100mm, 80 at 140mm and 50 at 215mm. A part pack often costs nearly as much as a full one, so check before you order a few short.

The default wastage is 5%. Raise it for walls with lots of cut blocks round windows, piers and returns.

• Not counted: wall ties, lintels, cavity trays, insulation, damp proof course, movement joints and padstones.
• Foundations: H+H's foundation blocks are laid below ground without mortar in the vertical joints. Use the concrete calculator for the footing.
• Brick outer leaf: count it separately with the brick calculator.
• No prices. Ask the merchant, and ask whether the price is per block or per pack.

Common questions