How the calculation works
The calculator adds a 10 mm mortar joint to the length and height of the brick to find how many bricks cover one square metre of wall face. A 9 inch wall is two bricks thick, so it uses twice as many.
Mortar is the wall volume minus the volume of the bricks. Dry mortar is taken as wet volume × 1.33, plus 15% for frog filling and spillage. Cement and sand are then split by your mix ratio (1:4 or 1:6), with 1 bag = 50 kg and cement density 1,440 kg/m³.
For a common 230×115×75 mm brick this gives about 392 bricks and 1.4 bags of cement (1:6) per cubic metre of brickwork.