The formula
Wall volume = length × height × thickness. Each masonry unit's "effective volume" adds a mortar joint's thickness to all three of its own dimensions (since each unit sits with a joint on its sides, top, and end within the wall) — dividing the wall volume by this effective unit volume, then applying a wastage allowance for breakage and cutting, gives the number of units needed. The mortar volume itself is simply the wall volume left over after subtracting the units' own (non-joint-inflated) volume.
A worked example — brick vs block for the identical wall
A 5m × 3m wall, 0.23m (9 inches) thick, built with standard 190×90×90mm bricks and a 10mm mortar joint, with 5% wastage: needs 1,812 bricks and about 0.661 m³ of mortar. The exact same wall built with 390×190×190mm AAC blocks instead, same joint thickness and wastage: needs only 227 blocks and about 0.254 m³ of mortar — roughly 8 times fewer units and noticeably less mortar, for an identical finished wall size.
Why fewer, larger units isn't a structural downgrade
The dramatic drop in unit count reflects that each AAC block simply occupies far more volume than a brick — it's not that the wall needs less material overall, it's that each individual unit is much bigger. AAC blocks are a legitimate, widely used modern masonry material (lighter than clay brick, often better thermal insulation), not a "cheaper, weaker" substitute — the structural design load calculation for a wall depends on the specific material's engineering properties, not simply on how many pieces make it up.
Why fewer units also means less labor and mortar
Beyond the material count itself, fewer, larger units directly reduce labor time (fewer individual pieces to lay, align, and joint) and mortar consumption (fewer total joint surfaces across the wall) — both genuine practical advantages independent of the material's structural properties, and a big part of why larger block systems have grown popular for non-load-bearing and infill walls in modern construction.
Why the mortar joint size matters more than it might seem
Because the joint thickness is added to every side of every unit before dividing into the wall volume, a thicker joint noticeably reduces the effective unit count needed (each unit "claims" more wall volume including its joint) while increasing the mortar volume required — getting the joint thickness input right (matching your actual specified joint, commonly 10mm for brick, sometimes different for block systems) matters for an accurate material estimate, not just an approximate one.