Material quantities come from two numbers: the work quantity from the budget or BOQ, and the material coefficient per unit of work from the unit price analysis. Requirement = quantity × coefficient, then converted to the unit you buy in (bags, m3, pieces) with a small waste allowance.
Coefficients used in the example
| Work | Materials per unit | Note |
|---|---|---|
| 1 m3 of K-225 concrete | Cement 371 kg, sand 698 kg, gravel 1,047 kg, water 215 litres | Widely quoted figures from SNI 7394:2008; for important structures use a laboratory mix design |
| 1 m2 of 7.5 cm lightweight block wall | 8.4 blocks, 3 kg adhesive | See the AHSP article |
| 1 m2 of 1:4 plaster, 15 mm thick | Cement 6.24 kg, sand 0.024 m3 | Widely quoted figures from the SNI plastering analysis |
Example: a 6 × 8 m house
Quantities come from the example house in the BOQ article, with the structural concrete rounded to 5 m3.
| Material | Calculation | Requirement | Buying unit |
|---|---|---|---|
| Cement for concrete | 5 m3 × 371 kg = 1,855 kg | 1,855 kg | 37.1 bags of 50 kg |
| Sand for concrete | 5 × 698 kg = 3,490 kg ÷ 1,400 kg/m3 | 2.49 m3 | |
| Gravel | 5 × 1,047 kg = 5,235 kg ÷ 1,350 kg/m3 | 3.88 m3 | |
| Lightweight blocks | 94.11 m2 × 8.4 blocks | 791 blocks | 7.1 m3 (111 blocks per m3) |
| Block adhesive | 94.11 m2 × 3 kg = 282 kg | 282 kg | 7.1 bags of 40 kg |
| Cement for plaster | 188.22 m2 × 6.24 kg = 1,174 kg | 1,174 kg | 23.5 bags of 50 kg |
| Sand for plaster | 188.22 m2 × 0.024 m3 | 4.52 m3 |
Then add a waste allowance of about 3% to 5% and round up: 39 bags of cement for concrete, 25 for plaster, 8 bags of adhesive, 7.5 m3 of sand in total. The bulk density of sand and gravel varies by source; ask your supplier, since sand is often sold by the truck or by the m3.
Steps
- Take each work item’s quantity from the BOQ or take-off sheet, not from the budget total.
- Take the material coefficients from the same unit price analysis the budget uses.
- Multiply, then add up the same material across all work items (cement from concrete, plaster and blockwork).
- Convert to buying units: kg to bags, kg to m3 with the bulk density, m2 to pieces.
- Add the waste allowance once, then round up.
- Plan purchases to follow the work schedule, so materials do not sit on site too long.
Common mistakes
- Waste added twice: in the coefficient and again at the end.
- Cement worked out per item but not added up, so purchases are split and cost more.
- Sand calculated in kg and bought by the m3 without allowing for its bulk density.
- All materials bought at the start, then damaged or lost in storage.
The full guide is in How to build a construction cost report that catches overruns early. It has a free template (Excel or Google Sheets) and a checker that reads your own workbook.
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Common questions
How do I work out how much cement I need?
Work quantity times the cement coefficient per unit, added up across all work items, then divided by the bag weight.
How much cement is in 1 m3 of K-225 concrete?
The widely quoted figure is 371 kg, about 7.4 bags of 50 kg. For important structures use a laboratory mix design.
How much waste should I allow?
Usually 3% to 5%, but do not add it if it is already in the analysis coefficients.
How do I convert kg of sand to m3?
Divide by the bulk density of the sand, for example about 1,400 kg per m3. Ask your supplier for the exact figure.
References
- SNI 7394:2008 unit price calculation for concrete work (Indonesian): material coefficients for concrete work