What this calculator does
A mortar calculator estimates how much bedding mortar a brick or block wall will use, based on the size of the wall, the dimensions of the units being laid, and the thickness of the mortar joints between them. It is the natural companion to a unit-count calculator: once you know how many bricks or blocks a wall needs, this works out the mortar volume that goes around them, and how many bags that comes to at a given yield per bag.
The calculation treats each unit as sitting inside a slightly larger "gross" footprint that includes half the surrounding joint on each side, then subtracts the unit's own solid volume. What is left over, multiplied by the number of units in the wall, is the mortar volume. It is an estimate rather than an exact figure, since real joints, cut units and mortar loss on the board all vary a bit from the geometry used here.
The formula
The wall area is divided by the footprint of one unit plus its joint to get the approximate number of units. Each unit is given a gross volume (its length and height plus the joint thickness, times its depth) and a net volume (its own length, height and depth); the difference is the mortar volume that unit displaces. Multiplying that by the unit count, adding the waste allowance, and dividing by the mortar volume a single bag yields gives the bags required.
| Term | Meaning |
|---|---|
| Mortar volume | The total volume of mixed mortar the wall is estimated to use, in cubic metres. |
| Joint thickness | The thickness of both the horizontal (bed) and vertical (perpend) mortar joints between units. |
| Yield per bag | The volume of mixed, workable mortar one bag of dry mix produces once combined with water or sand; this varies by product, so it is left editable. |
The inputs explained
| Field | What to enter |
|---|---|
| Wall length (m) | The finished wall length, measured along the ground. |
| Wall height (m) | The finished wall height, from base to top. |
| Unit length (mm) | The length of one brick or block, measured along the wall. |
| Unit height (mm) | The height of one brick or block, as it sits in the course. |
| Unit depth (thickness) (mm) | The depth (thickness) of the unit through the wall, used to convert the joint footprint into a joint volume. |
| Mortar joint thickness (mm) | The mortar joint thickness used on both bed and perpend joints. |
| Mortar volume per bag (m³) | The volume of usable mixed mortar a single bag produces once mixed, from the bag or mix supplier's data. |
| Waste allowance (%) | An allowance for mortar lost on the board, in the mixer, or from imperfect joints. |
When to use it
Ordering mortar for a new wall
Working from the planned wall length, height and the unit size being used, this gives a bag count to order before work starts, rather than guessing and making repeat trips to the supplier.
Comparing joint thicknesses
A thinner joint uses less mortar per unit but slightly more units overall, and a thicker joint the reverse; running both joint thicknesses through the calculator shows which way the mortar total moves.
Checking a quoted bag count
If a tradesperson's estimate lists a bag quantity, recomputing it from the same wall dimensions and joint thickness is a quick sanity check before committing to an order.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How mortar volume changes with joint thickness
The same wall, laid with a range of mortar joint thicknesses.
| Joint thickness (mm) | Mortar volume | Bags required |
|---|---|---|
| 6 mm | 0.187 m³ | 11 |
| 8 mm | 0.243 m³ | 14 |
| 10 mm | 0.296 m³ | 17 |
| 12 mm | 0.347 m³ | 20 |
| 15 mm | 0.418 m³ | 24 |
How bag count scales with wall area
A range of wall lengths at a fixed 2 m height.
| Wall length (m) | Mortar volume | Bags required |
|---|---|---|
| 4 m | 0.148 m³ | 9 |
| 8 m | 0.296 m³ | 17 |
| 12 m | 0.445 m³ | 25 |
| 16 m | 0.593 m³ | 33 |
| 20 m | 0.741 m³ | 42 |
Questions
Does this replace the bricks or blocks needed calculator?
No, they answer different questions. The bricks or blocks needed calculator gives the unit count for a wall; this mortar calculator estimates the mortar volume and bags that go around those units, using the same wall and joint inputs.
Why does the mortar volume depend on unit depth?
The mortar joint runs along the full depth of the unit at the bed and perpend joints, so a thicker unit has more joint volume per length of joint, even at the same joint thickness.
How much waste allowance is reasonable?
Ten percent is a common starting point for hand-laid brickwork, covering mortar left on the board, spillage, and imperfect joints. Larger or more repetitive jobs can sometimes get away with less; fiddly work with lots of cuts often needs more.
Why is the yield per bag editable rather than fixed?
Mortar mix yield varies by brand, mix ratio and whether sand is bought separately, so there is no single correct figure. Check the yield quoted on the bag or by the supplier and use that figure here.
For the number of bricks or blocks themselves, see the bricks or blocks needed calculator. For brick course heights on the same wall, see the brick course height calculator.