Number of bricks or blocks
Each unit with its mortar joint covers (l + j) × (h + j) of wall face. One square metre therefore needs 1/((l + j)(h + j)) units per layer. Multiply by the net wall area (openings deducted) and by the number of units across the thickness, then add wastage for breakage and cutting – usually 3–5% for bricks and 5% for blocks.
Mortar, cement and sand
Wet mortar is the wall volume minus the volume of the units. Loose dry materials occupy about a third more than the wet mortar, so the wet volume is multiplied by 1.33 and a mortar wastage allowance is added. That dry volume is split by the mix ratio: for 1:6, one part in seven is cement. With cement at 1440 kg/m³, a 50 kg bag is about 0.0347 m³.
Worked example
A 10 m × 3 m one-brick wall (228×114×76 bricks, 10 mm joints) with 4 m² of openings: net area 26 m², 238 mm thick. Bricks per m² per layer = 1/(0.238 × 0.086) = 48.86, so 26 × 2 × 48.86 = 2541 bricks, or 2668 with 5% wastage. Wet mortar = 6.188 − 5.019 = 1.169 m³, dry = 1.169 × 1.33 × 1.10 = 1.711 m³. For 1:6 mortar that is 352 kg of cement (7.04, so 8 bags) and 1.47 m³ (52 cft) of sand.
FAQ
Why is the mortar volume so large for brick walls?
Small bricks have many joints – roughly 19% of a one-brick wall is mortar, against about 5–8% for blockwork.
Which mix should I use?
1:6 is common for load-bearing or internal brickwork, 1:4 for exposed or below-ground work. Follow your specification.