Block Wall Calculator

In short A block wall has three quantities and most take-offs only get two of them. The blocks come from the wall area divided by the area one block occupies WITH ITS JOINTS: a 40 by 20 cm block with a 1 cm joint occupies 41 by 21 cm, so it takes 11.61 blocks per square metre rather than the 12.5 the bare block size suggests — ignoring the joint over-orders by about 7.6%. A 10 by 2.5 metre wall is 25 m², which is 290 blocks exactly and 305 with a 5% waste allowance. THE MORTAR IS DERIVED HERE RATHER THAN ALLOWED FOR. The wall’s bounding volume is 5.0 m³ and the blocks themselves occupy 4.65 m³ of it, so the mortar is the 0.35 m³ left over. That is subtraction, and it needs no assumption about how many bed and head joints there are or how thick each one is. AND THE THIRD QUANTITY IS THE ONE THAT GETS FORGOTTEN AT ORDERING. A hollow block is only about 55% solid, so each 40 by 20 by 20 cm block has roughly 7.2 litres of void in its cores. Fill a quarter of them and that is 76 blocks and 0.55 m³ of grout — more than one and a half times the mortar in the entire wall. Core fill is routinely the largest single volume on a block job and it is routinely left off the order. There is a fourth number worth looking at: the courses. At 21 cm per course a 2.5 metre wall is 11.90 courses, and a fraction means the wall does not land on a course line. Something has to give — a thicker bed joint, a cut course, or a different height — so this page reports the fraction rather than rounding it out of sight. This is not the brick calculator. That one counts bricks and gives mortar as a rough per-square-metre allowance, which is the right approach for a solid unit. Hollow block needs the subtraction and the core fill, and those are what this page adds. Counts and volumes are exact for the sizes you enter; the 55% solid default is typical and varies by manufacturer, so take it off your supplier’s data sheet.