Mortar calculator

How much mortar do I need?

Work out the wall area, less doors and windows, and multiply by the mortar per m² for the wall. With standard 215 × 65mm bricks and 10mm joints:

By count: about 0.37m³ per 1,000 bricks and 0.07m³ per 100 blocks. These include an allowance for the frogs, perforations and spillage, the same method as our brick and block calculator.

Mortar mix ratios: which mix for which wall

Mortar is specified by designation, i to iv, or by strength class, M12 to M2. Marshalls lists them in order of their ability to accommodate movement: the weaker the mix, the more movement in the wall it can take, so the strongest mix is not automatically the best. Each designation can be made with lime or with a plasticiser, which does the same job of making the mortar workable.

Where a range is given, such as 1:1:5 to 6, the MPA says to use the lower sand figure with sand that has more fines in it and the higher figure with sand that has fewer. For outside walls, above and below the DPC, Marshalls recommends designation iii: 1:1:6, or 1:5 to 1:6 with plasticiser.

How much sand and cement do I need?

For 10m² of half-brick wall, with 5% waste, that is 0.231m³ of mortar. At 1:6 with plasticiser: 3 bags of cement and 421kg of building sand. At 1:1:6 with lime: 3 bags of cement, 38 litres of hydrated lime and 369kg of sand.

The working: the dry materials take up about 1.33 times the volume of the wet mortar, because they settle when water goes in. That dry volume is split by the mix's parts, with cement at 1,440kg per m³ and sand at 1,600kg per m³. Cement is about £8.50 a 25kg bag and sand £42 a bulk bag, as estimates you can change.

Sand and cement for a floor is a different mix: use the screed calculator. For concrete, use the concrete calculator.

Lime or plasticiser?

Both make the mortar workable, so it spreads under the trowel and fills the joints. Historic buildings were built with lime mortar; a plasticiser is added to cement and sand instead of lime. Marshalls warns not to overdose a plasticiser, or to use cheap alternatives such as washing up liquid.

Lime is shown by volume in litres, because a bag of hydrated lime varies by maker. Measure each part with the same bucket.

How much mortar for repointing?

Only the joints get new mortar. With standard bricks and 10mm joints, the joints are 17.2% of the wall's face (Marshalls puts it at "nearly 17%"). Multiply the area by that share and by how deep you rake out: 10m² raked out to 20mm takes 0.034m³, which at 1:1:6 with 10% waste is 1 bag of cement, 6 litres of lime and 60kg of sand.

On an old or listed building, South Kesteven's conservation guidance says the new mortar should be no stronger or harder than the masonry it bonds, which means lime mortar, and that old mortar should come out with hand tools only. It also advises repointing only once the mortar has weathered back as deep as the joint is wide. Speak to your council's conservation officer before starting on a historic wall.

How many bricks will a tonne of mortar lay?

The MPA's rule of thumb is that 1 tonne of cement, lime and sand mortar lays about 1,000 bricks or 600 blocks. By volume, 1m³ of mortar lays about 2,730 bricks in a half-brick wall. That is 16.7m² of wall per 1,000 bricks, at 10 blocks per m² for blockwork.