How to Calculate Bricks, Blocks and Mortar
Calculate bricks or blocks per square metre, deduct openings, add wastage and estimate mortar for UK walling with a worked example.
The Quick Method
For a first-pass material order, calculate the net face area of every wall, multiply by the number of masonry units per square metre, then add an allowance for cuts and breakages.
Net wall area = total wall area − openings
Units required = net wall area × units per m² × waste factor
For common UK materials laid with nominal 10mm joints, useful starting figures are:
| Material | Typical work size | Approximate units per m² | |---|---:|---:| | Standard clay brick | 215 × 102.5 × 65mm | 59–60 | | Standard concrete block | 440 × 100 × 215mm | 9.9–10 |
These are face-area quantities for one leaf. A cavity wall normally needs its brick outer leaf and block inner leaf calculated separately. A solid wall or different bond can use more units, so always price from the specified construction rather than doubling a number without checking the drawing.
The Masonry Calculator handles the wall area, openings, joint size and waste allowance in one pass.
Why Mortar Joints Matter
The unit alone is not the module. The mortar joint occupies part of every square metre, so a count based only on the brick face will be too high.
The Brick Development Association's brickwork dimensions guide uses the widely adopted 215 × 102.5 × 65mm UK clay brick with a nominal 10mm joint. That produces a coordinating module of 225mm long by 75mm high on the face.
The calculation is:
1,000,000 ÷ (225 × 75) = 59.26 bricks per m²
For a 440 × 215mm block with the same nominal joint:
1,000,000 ÷ (450 × 225) = 9.88 blocks per m²
Measure the actual product when matching unusual, handmade, reclaimed or imperial brickwork. Manufacturer tolerances, non-standard units and a different agreed joint can change both the setting-out and the final quantity.
Worked Example: Brick Outer Leaf and Block Inner Leaf
Imagine a 6m-long wall, 2.4m high, with one 1.2 × 1.2m window and one 0.9 × 2.1m door.
1. Gross wall area
6 × 2.4 = 14.4m²
2. Openings
Window: 1.2 × 1.2 = 1.44m² Door: 0.9 × 2.1 = 1.89m² Total openings = 3.33m²
3. Net wall area
14.4 − 3.33 = 11.07m²
4. Brick quantity
11.07 × 59.26 = 656 bricks before waste
Using a 10% estimating allowance:
656 × 1.10 = 722 bricks, rounded up to the supplier's pack quantity
5. Block quantity
11.07 × 9.88 = 110 blocks before waste 110 × 1.10 = 121 blocks
Add returns, piers, parapets and areas below openings separately if they were not included in the original measured rectangles. Do not deduct tiny service penetrations: the units around them are usually cut rather than removed from the order.
Choosing a Sensible Waste Allowance
There is no single percentage that suits every job. Ten per cent is a practical estimating default, but the real allowance depends on:
- the number of corners, reveals and openings;
- bond pattern and decorative detailing;
- how many cuts can be reused;
- the fragility and consistency of the selected unit;
- delivery access, storage and handling; and
- whether matching replacements will be available later.
A simple, repetitive wall may need less. A small extension with many openings, returns and specials may need more. Separate expensive special shapes from the general count rather than hiding them inside one percentage.
Estimating Mortar
Mortar demand changes with unit size, frog or perforation, joint profile, wall thickness, wastage and the brick's texture and suction. That makes a site-specific yield or supplier coverage figure more useful than one universal mix rule.
As a working allowance, the Builder Tools calculator uses about 25kg of dry mortar per m² for a single leaf of standard brickwork and 8kg per m² for standard blockwork, then rounds up to complete 25kg bags. Treat that as an order check, not a mortar specification.
The mortar itself must suit the structural design, masonry unit and exposure. The BDA's Mortar for Brickwork guide explains why strength, water resistance, workability and joint profile all matter. Follow the designer's specification and the product instructions; do not strengthen a mix on the assumption that stronger is always better.
Checks Before Ordering
- Confirm the latest drawing revision and wall build-up.
- Measure every wall section and return separately.
- Deduct meaningful openings, not every small hole.
- Confirm the exact unit work size and intended joint.
- Add specials, closers, lintel bearing details and movement-joint materials.
- Apply a job-specific waste allowance.
- Round to full packs and check the merchant's return policy.
- Keep enough matching facing bricks for later damage or alterations.
Ready to run the wall? Enter the dimensions in the Masonry Calculator, then add the calculated materials to a professional quote.