Measure the net geometry first, use the real unit module or product yield, and apply a documented allowance only at the end.
1. Divide the work into consistent measured zones
Group only walls that use the same unit, bond and joint pattern. Group only floor zones that use the same screed product and thickness range. Sketch each zone and label dimensions, openings, steps and changes in level.
Compare drawing dimensions with the available site before ordering. Out-of-square walls, uneven bases and late service openings change quantities and may also require a design response.
2. Calculate masonry from the repeating face module
Subtract genuine openings from gross wall area, then divide by the face module formed by one unit plus its average vertical and horizontal joints. The body dimensions alone overstate the quantity when mortar joints are present.
Corners, returns, piers, bond cuts, lintels, reinforcement and special units need a course layout or separate take-off. Confirm wall thickness, strength, exposure class and structural details independently.
3. Use thickness measurements, not one optimistic point
Convert millimetres to metres and multiply net floor area by average thickness. Establish the average from level measurements while checking that every location remains within the product system’s permitted thickness.
The calculator converts order volume to bags using declared mixed yield. Do not estimate yield from dry bag mass: density, aggregate grading, water and compaction differ between products.
4. Keep allowance and supplier rounding visible
Add a stated allowance after the net quantity. Its purpose may include cutting, breakage, level variation or handling, but it should not conceal uncertain dimensions or unsuitable work.
Round masonry to full packs or pallets only after checking supplier quantities. Round screed to whole bags and confirm batch size, working time, storage and delivery limits.
5. Verify the design conditions before installation
Quantity does not establish wall stability, foundations, lintels, reinforcement, fire or moisture performance. It also does not establish screed load capacity, substrate preparation, bonding, movement joints, curing or floor-heating procedure.
Use the issued design, product data and applicable rules. Record the final quantities, product batches and site changes so later maintenance does not depend on an unexplained order total.
Keep net quantity and ordering decisions separate
| Material | Net basis | Add after calculation |
|---|---|---|
| Masonry units | Net wall area ÷ face module | Cuts, breakage and pack rounding |
| Special units | Course and detail take-off | Supplier pack rules |
| Screed volume | Net area × average thickness | Level variation and handling |
| Screed bags | Order litres ÷ declared yield | Whole-bag rounding |
Worked example
One wall and one floor zone
The wall is 10 m × 2.7 m with 4.2 m² openings and 400 × 200 mm modular face. The floor is 8 m × 5 m with 2 m² excluded and 50 mm average screed.
Wall net area = 27 − 4.2 = 22.8 m² Units = 22.8 / (0.4 × 0.2) = 285 Floor net area = 40 − 2 = 38 m² Screed volume = 38 × 0.05 = 1.90 m³
Apply separate justified allowances, then round to the actual pallet or bag quantities. Confirm structural details and product limits before work.
What to have ready
- Dimensioned zone sketch and verified site measurements
- Unit body size, joint pattern and special-unit schedule
- Screed levels, thickness limits and product yield
- Explicit allowance and supplier pack quantities
- Issued design, product data and installation plan
Common mistakes
- Counting masonry from unit body dimensions without joints.
- Subtracting openings that still require cut units and returns.
- Using one screed depth where the base varies.
- Converting bag mass directly into mixed volume.
Reference check: 5 September 2026. Sources include US public agencies. Principles are general; regulations and design values must be checked where you live.Worked examples use stated hypothetical inputs.