Mortar Brickwork
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Mortar volume depends on the wall's total area, the size of the mortar joints (typically 3/8 to 1/2 inch), and the number of brick courses and head joints — more joint area per brick (thinner bricks, wider joints) means more mortar per square foot of wall.

Joint thickness has an outsized effect on mortar volume because joints run continuously along every brick edge — increasing joint width from 3/8 in to 1/2 in can raise total mortar volume by 30% or more across a large wall, even though the change per joint looks small.

A common allowance is 10% to account for spillage, mixing losses, and mortar left in the mixer or on the board at the end of a batch. Underordering mortar mid-project often means a visible color or texture mismatch when a new batch is mixed to finish the job.

No — the volume needed depends on wall geometry and joint size regardless of mix type, since Type N, S, and M differ in their cement-to-lime-to-sand ratio and resulting strength, not in how much total mortar fills a given joint volume.

It's a standard trade convention because brick sizes are fairly consistent, so mortar-per-100-bricks tables (adjusted for joint thickness) let masons quickly estimate material needs once they know the total brick count, without recalculating wall geometry from scratch.