Two-leaf / cavity-wall quantities

This works out separate orders for the outer and inner leaves of a cavity wall, each from its own area and its own units per square metre. Use it when the two leaves are built in different units, typically brick outside and block inside.

For each leaf i, A_i is its net area in m² and N_i is how many units it takes per m² (for example about 60 for bricks or 10 for standard blocks). w is the waste percentage, applied to both leaves. Q_i is the number of units to order for that leaf.

Net area times units per m² gives the bare count; (1 + w/100) adds the waste; the ⌈ ⌉ brackets round up to whole units. Each leaf is rounded on its own because they are separate orders.

With 10 m² each side, 60 bricks/m² and 10 blocks/m², 5% waste gives 630 bricks and 105 blocks.

Watch out: the cavity itself is not masonry. The areas of the two leaves can differ because of returns, openings and closers, so measure each one.