Mortar Calculator
Dry volume is a third more than wet. That is the whole thing.
Work out Mortar. Dry volume is a third more than wet. That is the whole thing. States the assumption instead of hiding it.
Cement needed
8 bags
0.495 m³ of sand · 0.618 m³ dry mix for 24 m² of wall
Dry materials occupy about a third more volume than the mortar they make. Water fills the voids between grains rather than adding to them, so 1 m³ of mortar needs roughly 1.33 m³ of dry sand and cement — which is why ordering by wet volume leaves you a bag short on the last mix. The mix ratio is by volume, not weight, which is how it is measured on site with a bucket or a shovel. A 1:4 suits most brickwork above the damp course; below ground or in exposed positions a 1:3 is stronger, and soft historic brickwork wants 1:6 or a lime mortar so the mortar fails before the brick does. The mortar volume is the wall volume minus the volume the units occupy. Working it as bed joints plus perpends separately double-counts every intersection and overstates the answer by around 10%.
How the Mortar Calculator works
Cement, sand and water for a masonry wall, calculated from dry volume rather than wet. Dry materials occupy about a third more bulk than the mortar they make, which is why ordering by wet volume leaves you a bag short.
Also known as: how many bags of cement for a wall · sand and cement for bricklaying · mortar mix ratio for brickwork · how much sand per square metre
Frequently asked questions
How much mortar do I need per square metre?
Roughly 0.02 to 0.03 m³ of wet mortar per square metre of half-brick wall, depending on joint thickness and unit size. That needs about 0.03 to 0.04 m³ of dry materials, because of the bulking.
Why is dry volume more than wet volume?
Water fills the voids between sand grains rather than adding to the bulk. Roughly 1.33 m³ of dry sand and cement makes 1 m³ of mortar, and ignoring that is the commonest estimating error in small works.
What mix should I use?
1:4 cement to sand for most brickwork above the damp course, 1:3 below ground or in exposed positions, and 1:6 or a lime mortar for soft historic brick — where the mortar should fail before the brick does.
Is the mix by weight or volume?
By volume, measured with a bucket or a shovel on site. That is why the ratios are quoted as 1:4 rather than in kilograms, and why a consistent gauge box matters more than precision scales.
How much water goes in?
About half the cement weight, but adjust by feel — mortar should hold its shape on the trowel without slumping. Too much water weakens it substantially, and it is the easiest thing to get wrong on a hot day.
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