Show the formula and working
This mortar calculator works out how much bricklaying mortar you need for a brick or block wall, then splits it into bags of cement and tonnes of sand for mixing on site, or bags of ready mixed mortar. It is built for UK and European walls measured in square metres, with a feet and inches switch, and it starts from the real joint volume of your bricks or blocks rather than a vague rule of thumb.
Quick answer: The joints of a half brick wall of UK standard bricks hold about 17.6 litres of mortar per square metre. With 25% extra for frogs and waste, 10 m² needs about 220 litres. Mixed 1:5, that is about 67 kg of cement and 0.38 tonnes of sand. The mortar calculator shows each step.
What Is a Mortar Calculator?
A mortar calculator is a tool that estimates the volume of mortar needed to lay a wall and the cement and sand that make it. The joint volume per square metre is the wall thickness minus the volume of the bricks or blocks in that square metre. Multiply by the wall area and add an allowance for frogs, waste and droppings to get the wet mortar. Multiply by a dry to wet factor to get the dry materials, then split them by the mix ratio.
For example, a 1:5 mix is 1 part cement to 5 parts sand by volume, so one sixth of the dry volume is cement and five sixths is sand. Cement weight is its volume x its bulk density. One cement maker’s data sheet gives a settled bulk density of 1,300 to 1,450 kg/m³, so the calculator uses 1,400 kg/m³ as an editable default.
How Do You Use the Mortar Calculator?
- Choose the Masonry unit: a UK standard brick, a 100 mm or 140 mm block, or a custom size entered in Custom unit length, Custom unit height and Custom unit width.
- Enter the Wall area and choose the Wall build: one leaf, or two leaves with a filled collar joint.
- Check the Mortar joint and set the Extra for frogs, waste and droppings.
- Pick the Mix (cement to sand) your builder or designer specifies.
- Adjust the Dry to wet volume factor, Cement bulk density, Sand bulk density and Cement bag size if you have better figures.
- If you prefer premix, enter the Ready mixed mortar per bag (from the bag).
- Read the mortar volume, cement bags and sand, then open “Show the formula and working” to check each step.
How Much Mortar Do You Need Per Square Metre?
A half brick wall of UK standard bricks with 10 mm joints holds 17.6 litres of mortar per square metre in its joints, and a 100 mm block wall holds 6.6 litres. Blocks need much less because there are fewer joints per square metre. Real use is higher than the joint volume, so add an allowance: 25% is the default, and more for frogged bricks laid frog up.
| Wall | Units per m² | Mortar in joints per m² (no allowance) | With 25% allowance |
|---|---|---|---|
| Half brick, UK standard brick | 59.3 | 17.6 litres | 22.0 litres |
| One brick, UK standard brick | 118.5 | 45.2 litres | 56.5 litres |
| 100 mm block, 440 x 215 mm | 9.88 | 6.6 litres | 8.2 litres |
| 140 mm block, 440 x 215 mm | 9.88 | 9.2 litres | 11.5 litres |
| 215 mm block, 440 x 215 mm | 9.88 | 14.1 litres | 17.6 litres |
All figures assume 10 mm joints. Bricks and blocks follow the UK sizes in the Wikipedia article on bricks and the article on concrete masonry units.
What Mortar Mix Ratio Should You Use?
Use the mix your designer, builder or brick maker specifies, because the right strength depends on the bricks, the exposure and the job. The prescribed mixes for masonry mortar link cement and sand ratios to strength classes such as M12, M6, M4 and M2, set out in the MPA Mortar data sheet on mortar classes. A stronger mix is not always better: mortar that is harder than the bricks can make them crack instead of the joint.
Sand changes the result too. Soft building sand is normal for bricklaying, and the bulk density of damp sand differs from dry sand, which is why the sand density is an editable input.
How Many Bricks Does a Bag of Mortar Lay?
One maker’s data sheet for an M6 ready mixed mortar gives a yield of 11 litres for its larger MaxiPack 20 bag and says it lays about 19 standard bricks. The joint volume alone suggests more, about 30 bricks per bag at a 25% allowance, because the maker’s figure includes filled frogs and generous bedding. If you want to match a maker’s figure, raise the allowance; if you are an experienced bricklayer using solid bricks, the lower figure may be enough.
Worked Example: Garden Wall Mortar
Example: 10 m² of half brick wall is built in UK standard bricks with 10 mm joints. Allow 25% extra for frogs, waste and droppings, mix 1:5 with a dry to wet factor of 1.3, and use 1,400 kg/m³ for cement, 1,600 kg/m³ for sand and 25 kg cement bags.
- Joint mortar per m²: 102.5 mm wall minus 59.26 bricks of 215 x 65 x 102.5 mm = 17.6 litres.
- Wet mortar: 10 m² x 17.6 litres x 1.25 = 220 litres, or 0.22 m³.
- Dry materials: 0.22 m³ x 1.3 = 0.286 m³.
- Cement: 0.286 / 6 = 0.048 m³ x 1,400 kg/m³ = 67 kg, so 3 bags of 25 kg.
- Sand: 0.286 x 5 / 6 = 0.239 m³ x 1,600 kg/m³ = 382 kg, about 0.38 tonnes.
If you bought premix instead, 220 litres at 11 litres per bag would be 21 bags. When the wall is finished, you can work out the paint for it with the masonry paint calculator. More tools are in our home calculators.
Mortar Calculator FAQs
How much mortar do I need for 1,000 bricks?
The joints around 1,000 UK standard bricks in a half brick wall hold about 0.3 m³ of mortar, or about 0.37 m³ with a 25% allowance. That is 1,000 bricks divided by 59.3 per m², about 16.9 m², times 22 litres per m².
How many bags of cement do I need for mortar?
For 10 m² of half brick wall at 1:5, you need about 67 kg of cement, or 3 bags of 25 kg, with 25% allowance. Richer mixes like 1:4 use more cement, and weaker mixes like 1:6 use less. The calculator works it out for your mix.
What is the dry to wet volume factor?
Dry cement and sand take up more space than the wet mortar they make, because water fills the gaps between grains. The calculator multiplies the wet volume by 1.3 by default to estimate the dry materials. It is an editable example, not a fixed constant.
How much does a cubic metre of sand weigh?
It depends on the sand and how damp it is, so there is no single figure. The calculator uses 1,600 kg/m³ as an example default. Ask your supplier for their figure, especially when you order sand by the tonne rather than by volume.
Is ready mixed mortar better than mixing my own?
Ready mixed bags give a consistent mix and suit small jobs and repairs. Mixing cement and sand on site costs less per litre for larger walls but needs care to keep every batch the same, so the mortar colour and strength stay even.
Why does my wall use more mortar than calculated?
Filled frogs, thick bed joints, mortar dropped while laying and mortar left in the bucket all add up. Measure what one bag or batch actually lays on the first courses, then raise the allowance in the calculator to match.
Checked October 2026. Sources: MPA Mortar (Mineral Products Association): Data Sheet 02, mortar classes and prescribed mixes, Breedon: Portland cement data sheet (bulk density), Heidelberg Materials: Sand and cement mortar data sheet (yield), Wikipedia: Brick.