To calculate mortar for a brick or block wall, you need the wall dimensions, masonry unit size, joint thickness, openings, mortar mix ratio, and an allowance for waste. The result tells you the approximate volume of mortar required and the amount of cement and sand needed for the specified mix.
The most reliable approach is to calculate the masonry first, estimate the mortar volume from the joints, then convert the wet mortar requirement into dry material quantities.
Table of Contents
- What You Need to Calculate Mortar
- 1. Calculate the Wall Area
- 2. Deduct Doors and Windows
- 3. Calculate Brick or Block Quantity
- 4. Calculate Mortar Volume
- 5. Convert Wet Mortar to Dry Materials
- 6. Apply the Cement-to-Sand Ratio
- 7. Calculate Cement Quantity
- 8. Calculate Sand Quantity
- Worked Brickwork Example
- Worked Blockwork Example
- Add a Practical Waste Allowance
- Why Actual Mortar Use Varies on Site
- Common Mortar Calculation Mistakes
- Use the Mortar Calculator
- Frequently Asked Questions
What You Need to Calculate Mortar
Before starting, collect these measurements:
- Wall length
- Wall height
- Wall thickness
- Brick or block dimensions
- Mortar joint thickness
- Number and size of openings
- Specified cement-to-sand ratio
- Waste allowance
For a simple wall, the calculation starts with the wall area. For a more detailed estimate, the volume occupied by the masonry units and the mortar joints is also considered.
1. Calculate the Wall Area
Start by finding the gross wall area before deducting openings.
For example, a wall measuring 8 m long and 3 m high has:
So the gross wall area is 24 m².
If the wall contains several sections, calculate each section separately and add the areas together.
For more help with wall measurements, use the Wall Area Calculator.
2. Deduct Doors and Windows
Doors, windows and other openings reduce the amount of masonry required. They also reduce the mortar required in the wall area occupied by those openings.
Suppose the 24 m² wall has:
- One door measuring 0.9 m × 2.1 m
- Two windows measuring 1.2 m × 1.2 m each
The door area is:
The two windows occupy:
Total openings:
Net wall area:
The masonry and mortar estimate should now be based on approximately 19.23 m² rather than the full 24 m².
3. Calculate Brick or Block Quantity
The number of masonry units provides another way to estimate mortar consumption. Each brick or block creates bed and head joints, so unit dimensions and joint thickness have a direct effect on mortar use.
For a simple estimate, calculate the face area of one unit including the mortar joint.
Then:
This method is useful when the brick or block dimensions and joint thickness are known.
4. Calculate Mortar Volume
There are two practical approaches to estimating mortar volume.
Method 1: Wall volume minus masonry volume
For solid masonry, calculate the wall volume first:
Then calculate the volume occupied by the masonry units:
The theoretical mortar volume is:
This approach works well for solid bricks when the unit dimensions and quantity are known accurately.
Method 2: Estimate from joint volume
For detailed masonry work, mortar can be estimated from the bed joints and head joints between individual units.
This approach becomes more useful when joint thickness, unit dimensions and the laying pattern are known. It also avoids treating the entire external block envelope as solid material when hollow blocks are involved.
5. Convert Wet Mortar to Dry Materials
Mortar is measured as a wet mixture after cement, sand and water are combined. Cement and sand are purchased as dry materials, so a conversion is required when estimating material quantities.
A common estimating allowance is to multiply the wet mortar volume by approximately 1.33 to account for the reduction in volume after dry materials are mixed.
This is an estimating factor, not a universal constant. Local specifications, sand moisture, material grading, mixing method and site practice affect actual consumption.
6. Apply the Cement-to-Sand Ratio
The mortar mix ratio determines how the dry material volume is divided between cement and sand.
For an example using a 1:5 cement-to-sand ratio, the total number of parts is:
Cement therefore represents:
Sand represents:
The specified mortar ratio should always come from the project requirements. Different applications use different mortar specifications.
7. Calculate Cement Quantity
Once the dry material volume is known, calculate the cement portion.
For a 1:5 mix:
To convert cement volume into bags, you need an assumed bag volume or cement density.
For estimating, a 50 kg cement bag is often treated as roughly 0.0347 m³ using an assumed bulk density of about 1,440 kg/m³.
The actual volume represented by a bag varies with packing and bulk density, so the bag figure should be treated as an estimate.
8. Calculate Sand Quantity
Sand takes the remaining portion of the dry mix.
For a 1:5 mortar mix:
Sand is normally ordered by volume, although local suppliers sometimes sell by weight. If purchasing by weight, use the bulk density supplied for the specific sand.
Worked Brickwork Example
Consider a solid brick wall with these dimensions:
| Item | Value |
|---|---|
| Wall length | 8 m |
| Wall height | 3 m |
| Wall thickness | 0.115 m |
| Net wall area | 24 m² |
| Example mortar ratio | 1:5 cement:sand |
For simplicity, assume the wall has no openings in this example.
Wall volume:
Now assume the brickwork calculation gives approximately 1,080 bricks, with each brick measuring 215 mm × 102.5 mm × 65 mm.
Brick volume is approximately:
The theoretical wet mortar volume is therefore:
Using the illustrative 1.33 dry-volume factor:
For a 1:5 mix, cement represents one-sixth of the dry volume:
Estimated 50 kg cement bags:
Sand represents five-sixths:
So this illustrative calculation gives approximately 8 bags of cement and 1.34 m³ of sand before adding a site waste allowance.
Worked Blockwork Example
Blockwork requires extra care because blocks are often hollow. A gross block size does not represent the volume of solid block material.
Suppose a wall has:
- Area: 30 m²
- Block size: 400 mm × 200 mm face dimensions
- Mortar joints: 10 mm
- Example mortar ratio: 1:6 cement:sand
The approximate face coverage of one block including the joint is:
Estimated block quantity:
Allowing for the practical purchase quantity, this is about 349 blocks before adding any separate allowance for breakage or cutting.
Mortar should then be estimated from the actual bed and head joints, block configuration and project practice. For hollow blocks, use the block manufacturer's dimensions and the project's mortar consumption data where available.
Add a Practical Waste Allowance
Some mortar is lost during mixing, handling, transfer and laying. Mortar also remains on tools, boards and containers.
A common estimating allowance is around 5% to 10%, depending on site conditions and workmanship.
For a 5% allowance:
For a 10% allowance:
Waste should be applied consistently. If you add the allowance to mortar volume, avoid adding the same allowance again to cement and sand unless your estimating method specifically requires it.
Why Actual Mortar Use Varies on Site
Two walls with the same dimensions can consume different amounts of mortar.
The main reasons include:
- Joint thickness: thicker joints require more mortar.
- Unit size: smaller bricks create more joints over the same wall area.
- Unit tolerance: irregular masonry units often require more mortar for alignment.
- Workmanship: inconsistent bedding can increase consumption.
- Wall openings: doors and windows reduce masonry area.
- Sand condition: moisture and grading affect batching and volume.
- Block geometry: hollow blocks require a different material-volume approach.
- Site losses: handling and leftover mortar affect actual usage.
- Mix specification: the cement-to-sand ratio changes material quantities.
For estimating work, keep the theoretical calculation separate from the final procurement quantity. The first shows the calculated requirement. The second includes practical allowances.
Common Mortar Calculation Mistakes
Using the gross wall area
Ignoring doors and windows increases the masonry and mortar estimate.
Ignoring joint thickness
Mortar consumption depends heavily on the size and number of joints.
Using brick volume for hollow blocks
The external dimensions of a hollow block include its cores. Treating the full envelope as solid material produces a misleading mortar estimate.
Assuming one mix ratio works everywhere
Mortar ratios depend on the specified application and project requirements. Use the specified mix rather than choosing a ratio from a generic table.
Confusing wet volume with dry material volume
Cement and sand do not retain the same combined volume after mixing. Apply the appropriate estimating factor.
Forgetting waste
A theoretical quantity does not account for normal handling and site losses.
Rounding too early
Keep several decimal places during the calculation and round the final cement bags or purchase quantities at the end.
Use the Mortar Calculator
Need a faster estimate?
Use the Mortar Calculator to work through the mortar quantity for your masonry project.
For a complete estimate, you can also check the Brick Calculator, Block Quantity Calculator, and Wall Area Calculator.
Quick Mortar Calculation Reference
| Calculation | Formula |
|---|---|
| Wall area | Length × Height |
| Net wall area | Gross area − Opening area |
| Wall volume | Net wall area × Wall thickness |
| Masonry volume | Number of units × Unit volume |
| Wet mortar | Wall volume − Masonry volume |
| Dry material estimate | Wet mortar × Applicable dry-volume factor |
| Cement volume | Dry volume × Cement parts ÷ Total parts |
| Sand volume | Dry volume × Sand parts ÷ Total parts |
| Final quantity | Calculated quantity × (1 + Waste allowance) |
Frequently Asked Questions
How do I calculate mortar for a brick wall?
Calculate the net wall area, determine the brick quantity, estimate the mortar volume from the wall and brick volumes or joint dimensions, convert wet mortar to dry material volume, then divide the dry volume according to the specified cement-to-sand ratio.
How much mortar is needed for blockwork?
The quantity depends on block dimensions, joint thickness, wall area, block configuration and workmanship. Hollow blocks need special treatment because their gross external volume is not the same as their solid material volume.
What is the formula for mortar volume?
For a simplified solid-masonry calculation, mortar volume equals wall volume minus the volume occupied by the masonry units. A joint-based calculation can also be used when the unit dimensions and joint thickness are known.
What does a 1:5 mortar mix mean?
A 1:5 cement-to-sand mix means one part cement is combined with five parts sand by the specified batching method. The project specification should determine the required ratio.
How do I convert wet mortar to cement and sand?
First estimate the wet mortar volume. Apply the appropriate dry-material conversion factor, then divide the resulting dry volume according to the cement-to-sand mix ratio.
Should I add waste to mortar calculations?
Yes. A practical allowance is often added for handling, mixing and laying losses. A common estimating range is about 5% to 10%, but the appropriate allowance depends on site conditions and project practice.
Does mortar joint thickness affect quantity?
Yes. A thicker joint increases mortar consumption. Smaller masonry units also create more joints over the same wall area.