Mortar Calculator

How much mortar per m² of brickwork and blockwork

Makers give the mortar volume for each brick or block. We multiply it by the bricks or blocks in one square metre of wall. Travis Perkins counts about 60 bricks per m² in a single brick wall. Kilsaran counts 9.88 blocks of 440 × 215 mm.

Bricks: Blue Circle, Heidelberg and Cemex. Blocks: Blue Circle to Kilsaran. Bags = litres ÷ 12.

Why the block figures differ

Blue Circle's block figure matches what the joints hold (our sum), so it allows nothing spare. Kilsaran, an Irish maker, allows almost twice as much. 3 of the 5 figures we found sit near 1 litre (0.83 to 1.11). So the calculator uses Cemex's figure, 1 litre per block.

Mortar mix ratios and classes (M4, M6, M12)

A mix is parts of cement to parts of building sand. The class tells you how strong it is. A 1 : 3 mix has the most cement. The BDA, the brick trade body, says it is very strong but brittle.

Classes from Heidelberg's cement data sheet and the MPA. Uses as the makers sell them.

For everyday bricklaying, Gilmore Building Supplies names 1 : 5. Screwfix suggests a 4 to 1 general mix for standard walls.

Garden walls that stand alone, and walls that hold back soil, need sulfate-resisting cement, says the BDA. So do walls on sulfate soils, such as London clay. For any wall that holds weight, check your local building rules or ask a builder or engineer.

What goes in mortar: sand, lime, plasticiser and water

• Sand: use building sand, also called soft sand. One bulk bag of it lays about 900 bricks, by Heidelberg's figure.
• Lime: the BDA's 1 : 1 : 6 mix of cement, lime and sand fills the gaps well. The lime helps it stick and flex. Use hydrated lime to BS EN 459-1.
• Plasticiser: weaker mixes need lime or a plasticiser to stay soft enough to work. Follow the dose on the label. The BDA says not to use washing-up liquid.
• Water: Cemex adds 3.5–4 litres to each 25 kg bag of its dry mortar. Use it within 2 hours.
• Cold: stop when it is 2 °C and falling, says the BDA. Cover new work to keep frost off.

Repointing: how deep to rake out

SPAB is a charity that looks after old buildings. It says to rake joints out to twice their width, and deeper where joints are wide. So a 10 mm joint comes out about 20 mm deep.

On an old building, SPAB says the mortar will normally be lime. It advises against adding cement.

The BDA says about 17.5% of a brick wall is mortar. So raking 20 mm deep takes out about 3.5 litres per m². One 25 kg bag of mortar mix would refill about 3 m². That is our rough sum, not a maker's figure.

Bagged mortar or mix your own?

Bagged mortar, or premix, has the sand and cement in it already. Cemex sells 25 kg bags of M4 and M12 dry mortar. Its coverage: one bag lays about 20 bricks. Heidelberg's premixed 20 kg bag makes 0.011 m³. Hanson's 2021 guide lists M6 bags at 60 a pallet, marked not on sale.

We have no checked UK price for bags of mortar mix yet. Cement is about £8.39 a 25 kg bag, and building sand £67.35 a bulk bag, with VAT. With only 3 sellers each, these are a rough guide.

No UK maker gives a figure for wasted mortar, so the calculator starts at 0%. If you want spare mortar, choose 5% or 10%.

For a full shopping list, use our sand and cement calculator. To start from a count, try the brick mortar or block mortar calculator.