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Technical guideMaterial planning
From measured quantity to purchase quantity
Waste factors, unit conversion and rounding each change what you order. Apply them in a fixed order and write each one down.
Vương Phạm · Published · 4 min read
The quantity you measure is not the quantity you buy. Between the two sit three adjustments: material conversion, waste and rounding to purchasable units. Most ordering mistakes come from applying them in a different order each time, or not recording them at all.
Apply adjustments in a fixed order
- Measured work: the net quantity from dimensions, for example m³ of concrete.
- Material demand: convert work to materials using a named norm, for example kg of cement per m³.
- Waste: multiply by a stated factor for handling and site loss.
- Purchase quantity: round up to the unit a supplier sells, such as whole bags.
Watch out
Rounding before applying waste understates the order. Rounding every component separately before summing overstates it. Sum first, then round once per material.
Worked example
| Step | Calculation | Result |
|---|---|---|
| Measured work | 4.0 × 5.0 × 0.12 | 2.40 m³ |
| Cement demand | 2.40 m³ × 320 kg/m³ (example norm) | 768 kg |
| Waste | 768 kg × 1.03 (example factor) | 791 kg |
| Purchase | 791 kg ÷ 50 kg per bag, rounded up | 16 bags |
Record the assumptions
Next to every purchase quantity, write the norm source, waste factor, rounding rule and the date of the price used. When the order is checked or repeated, the reviewer can see exactly which assumption to change.