The calculation
Adding the mortar joint thickness to both the length and height of the unit before multiplying gives the true centre-to-centre footprint each brick or block occupies on the wall face, including its share of the surrounding mortar. Multiplying that rate by your wall area, then adding a wastage allowance, gives the total unit count.
Reference sizes
Two commonly used sizes: a standard UK brick (215×65mm face with a 10mm joint) works out to roughly 60 units per m². A common Pakistani concrete block (400×200mm face) comes to roughly 12.5 units per m². Because the formula is geometric rather than a lookup table, it holds for any brick, block, or AAC panel size you enter.
Frequently asked questions
What wastage allowance should I use for blockwork?
5% is a reasonable default for straightforward walls. Add more for walls with lots of openings, corners, or cut courses.
Does this account for openings like doors and windows?
No — enter the net wall area (gross area minus openings) for an accurate count.
Why does mortar joint thickness matter so much?
Because it's added to both dimensions of the unit before the area calculation, joint thickness has an outsized effect — a wall built with a 15mm joint instead of 10mm can need noticeably fewer units per m², since more of the wall's area is mortar.