Mortar is what is left over
The reliable way to work out mortar is not a rule of thumb per square metre. It is subtraction:
mortar volume = wall volume − the solid volume of the bricks in it
Take the wall's face area, multiply by its thickness to get total volume. Work out how many bricks it contains, multiply by the volume of one brick, and take that away. Whatever remains is mortar, because nothing else is in there.
This is worth doing properly because it self-corrects. A wider joint means fewer bricks, which means less brick volume subtracted, which means more mortar — automatically, without a second rule. Any method that treats brick count and mortar quantity as independent will drift.
A worked example
A wall 10 m long and 2.4 m high, single leaf, standard 203 × 57 mm bricks, 10 mm joints:
- Face area: 24 m²
- Wall thickness: 92 mm, so total volume 24 × 0.092 = 2.21 m³
- Bricks: each occupies 213 × 67 mm of face, so 24 ÷ 0.01427 = about 1682 bricks
- Solid brick volume: 1682 × (0.203 × 0.057 × 0.092) = 1.79 m³
- Mortar: 2.21 − 1.79 = 0.42 m³
So roughly 0.42 cubic metres of mortar for 24 square metres of wall — about 17 litres per square metre. That is the sort of figure worth carrying in your head as a sanity check on any estimate.
Dry materials bulk more than the mix
Like concrete, mortar shrinks when mixed. Sand in a heap is full of air, and the cement and water fill those voids. The usual allowance is 1.30, so 0.42 m³ of mortar needs about 0.55 m³ of dry cement and sand.
Split that by the mix ratio. For a 1:4 mix, five parts total:
- Cement: 0.55 ÷ 5 = 0.11 m³. At 1440 kg/m³ that is 158 kg, or four 40 kg bags.
- Sand: 0.55 × 4/5 = 0.44 m³. At 1.6 tonnes/m³, about 0.7 tonnes.
The cement figure is the one people get badly wrong. Multiplying the whole mortar volume by the density of cement — as if the mortar were entirely cement — over-orders by roughly fourfold. Mortar is mostly sand.
Choosing the mix
The ratio is cement to sand by volume, and richer is not better:
- 1:3: Very strong, for engineering brick and below-ground work. Rarely needed above ground.
- 1:4: The general-purpose structural mix. Load-bearing walls, exposed elevations.
- 1:5: General walling in sheltered positions. A common compromise.
- 1:6: Soft, for internal blockwork and work with soft or historic bricks.
The principle is that **mortar should be weaker than the brick**. When a wall moves — and every wall moves — you want the cracking to happen in the joints, where it can be raked out and repointed. A mortar stronger than the brick makes the brick crack instead, and that is not repairable.
This is the single most common mistake in repointing old buildings: a modern cement mortar in a wall built with soft handmade brick and lime will destroy the brick faces within a few winters.
What changes the quantity on site
- Double wythe: Doubles both the bricks and the wall thickness, so roughly doubles the mortar.
- Frogged or perforated bricks: The frog fills with mortar in most laying practice, which adds materially to the quantity — the calculation above assumes a solid brick and will read low.
- Blockwork: Far fewer, larger units means much less joint per square metre, and much less mortar.
- Waste: Mortar dropped, mortar gone off in the barrow, the last of the mix. Ten per cent is a fair allowance and it is genuinely used up.