What this calculator works out
A block wall comes down to two shopping lists that trace back to the same wall area: the blocks themselves, and the mortar that beds and joints them. This tool takes a wall's length and height, or an area you've already worked out, an optional deduction for doors and windows, and a chosen block size, and turns both lists into a single set of numbers: blocks to buy with an honest wastage allowance, then the mortar underneath it as building sand in tonnes and cement in 25kg bags, plus a US-style ready-mix mortar bag count for anyone buying bagged mortar instead of separate sand and cement.
It works for a UK garden wall or a house extension in 440 × 215 × 100mm standard block, an American wall in a 16 × 8 × 8in CMU, or a custom block of any size, with every measurement carrying its own metric or imperial toggle so a wall measured in metres and a block bought in inches can sit side by side without converting by hand.
How the maths works
Openings come out of the wall first, since a door or window opening doesn't get any blocks. Whatever's left, the net area, is multiplied by the chosen block's coverage rate and rounded up, since no merchant sells part of a block.
A wastage percentage is then applied on top and rounded up again, the same two-step pattern this site's Paving and Block Paving calculators use, because rounding up once for the raw count and again for wastage keeps both steps honest rather than compounding a single rounded figure.
Mortar works from the same net area: a fixed volume per square metre of 100mm blockwork, split by the chosen cement:sand ratio into a sand volume and a cement volume, then each converted to what's actually bought, tonnes of building sand and 25kg bags of cement.
A worked example
Take a 6m by 2m UK garden wall, 12 square metres, no openings, built in 440 × 215 × 100mm standard block with a 5% wastage allowance for a straight run. At 10 blocks per square metre, 12 × 10 = 120 blocks needed; with 5% added for cuts and breakages, 120 × 1.05 = 126, rounded up to 126 blocks to buy. Mortar for that same 12 m² is 12 × 0.0125 = 0.15 m³; split 1:4, that's 0.12 m³ of building sand (0.192 tonnes at 1.6 t/m³) and 0.03 m³ of cement, which at 1,440 kg/m³ is about 43.2kg, rounding up to 2 bags of 25kg cement. At a rough £1.20 a block, 126 × £1.20 = £151.20 for the blocks alone, before sand, cement or labour.
The American aside: an 8ft by 40ft CMU wall, 320 square feet, built in standard 16 × 8 × 8in block with a 10% wastage allowance for corners and openings. At 1.125 blocks per square foot, 320 × 1.125 = 360 blocks needed; with 10% added, 360 × 1.1 = 396, rounded up to 396 blocks to buy. Using the default 12.5 blocks per 80lb bag of ready-mix mortar, that's 396 ÷ 12.5 = 31.68, rounding up to 32 bags. At a rough $1.75 a block, 396 × $1.75 = $693 for the blocks alone, again before mortar or labour.
Choosing a block system and size
440 × 215mm is the coordinating face size UK builders' merchants quote as standard, and it's genuinely near-universal: it already folds in a 10mm mortar joint on every side, which is why it lands on a clean 10 blocks per square metre rather than a figure the block's raw printed dimensions alone would give. The 100mm is the block's own depth through the wall, the dimension a "100mm block" or "140mm block" name actually refers to, not its face size.
A standard American CMU works the same way in imperial units: 16 × 8in is the nominal face size, already including a 3/8in mortar joint, giving 1.125 blocks per square foot (112.5 per 100 sq ft). The 8in in "8 × 8 × 16" is the block's own depth, matching a UK 100mm block reasonably closely in load-bearing single-leaf work, though the two aren't interchangeable products.
Custom block sizes use the same logic worked out directly: enter the block's own face length and face height (not its depth) and the mortar joint width, and this calculator adds the joint to both dimensions, multiplies them into an effective footprint, and divides a square metre by that footprint, the same effective-footprint method this site's Paving Calculator uses for slabs.
The mortar mix itself
1:4 (cement to building sand, by volume) is the standard mix most guides quote for dense concrete block; a leaner 1:5 mix is common for lighter aerated block, which doesn't need as strong a bed. Below-DPC work, where a wall needs extra resistance to rising damp and ground moisture, is usually mixed richer again, closer to 1:3, a stronger mix than either default here. The mortar volume itself, about 0.0125 m³ per square metre of 100mm blockwork, doesn't scale simply with a thicker or thinner block, since it mostly reflects the bed and perpend joint pattern across the wall face rather than the block's full depth; treat it as most accurate for blockwork close to 100mm thick and check a supplier's own coverage guide for anything markedly thicker.
Building sand, sometimes called soft or bricklaying sand, is what's specified here rather than the sharp sand this site's Paving Calculator uses for a slab bedding layer: building sand's finer, more rounded grains make a smoother, more workable mortar for laying and pointing, where sharp sand's coarser grains suit a structural bedding layer instead.
Cavity walls, wall ties and DPC
This calculator works out material for a single leaf, one skin of blockwork, the way a garden wall, a partition or the inner leaf of a cavity wall is usually specified. A full cavity wall has two leaves with a gap between them, an inner and an outer skin tied together rather than built solid, so a genuine cavity job needs this calculator run twice, or its figures simply doubled, once for each leaf.
Wall ties hold those two leaves together across the cavity so they act as one structural wall rather than two independent skins. BS 5628's standard spacing is roughly 900mm horizontally and 450mm vertically, staggered between courses so no tie lines up directly above the one below, closing in to around 750mm horizontal centres near openings, corners and movement joints where the wall needs more restraint. A damp-proof course (DPC) is a continuous impermeable layer built into every leaf at least 150mm above finished ground level, stepped up on sloping ground so it's never lower than that figure at the lowest point, stopping rising damp from working its way up through the blockwork from the ground.
Questions people ask
What's the difference between dense and aerated (aircrete) blocks?
Dense concrete block is heavier and stronger, the usual choice for load-bearing walls, foundations and below-DPC work. Aerated (aircrete) block is lighter, easier to cut and a better insulator, common for internal walls and the inner leaf of an insulated cavity wall. Both are laid to the same coordinating face sizes, so the blocks-per-m² figure stays the same either way; it's the mortar mix and structural use that differ, which is why this calculator offers a leaner 1:5 mix alongside the 1:4 standard.
Should blocks be laid flat or on edge?
Almost always flat, on their largest face, which is what the 440 × 215mm coordinating size and the 10 blocks per m² figure both assume. Laying a block on edge (on its narrower face) is unusual, mostly seen in specific detailing rather than general walling, and changes both the coverage rate and the effective wall thickness, so it needs its own working rather than this calculator's standard figures.
Do I need wall ties?
Only if the wall is a cavity wall with two separate leaves; a solid single-leaf wall doesn't need them. Where they are needed, BS 5628's standard spacing is roughly 900mm horizontally and 450mm vertically, staggered between courses, closing in nearer openings and corners. Tick "show wall tie and DPC spacing guidance" above for the figures alongside your own results.
100mm or 140mm block, which should I use?
Both are laid to the same 440 × 215mm face size and the same 10 blocks per m², since the difference is depth through the wall, not face area, so this calculator's block count doesn't change between them. 100mm is the common internal and inner-leaf thickness; 140mm and other thicker blocks are typically specified for load-bearing external work or where building regulations call for extra structural or thermal performance, worth confirming against your own project's structural drawings.
Why isn't the block count just wall area divided by block size?
Because the printed 440 × 215mm size already has a 10mm mortar joint folded into it on every side; dividing by the block's face area alone would undercount, since it ignores the joint each block actually takes up on the wall. That's also why UK merchants and blockwork guides quote 10 blocks per m² as a flat, ready-to-use figure rather than something to recalculate from scratch each time.
How much wastage should I allow?
5% suits a simple straight run with few cuts. 10% is the safer default once corners, piers, openings or a stepped foundation mean more blocks need cutting, since a cut block breaks more easily than a whole one and off-cuts rarely reuse cleanly elsewhere on the same job.
This calculator uses verified UK and US block coverage rates, checked against two independent sources each: Gilmore Building Supplies' merchant guide and EasyEstimate's UK Blockwork Calculator for the 10 blocks/m² UK figure; Inch Calculator's Concrete Block Calculator and the same 1.125 blocks/sq ft figure used by other published CMU calculators for the US rate. Mortar volume is checked against EasyEstimate's and materialcalculator.co.uk's mortar calculators; the US ready-mix bag default is the mid-point of Quikrete's and Sakrete's own Mortar Mix product data sheets. Real coverage varies with block batch, joint thickness and site conditions, so treat the figures here as a solid ordering estimate and confirm against your own supplier's data before a large order.