In this example, increasing net quantity needed from 86 to 94.6 changes the result from 8 packs to 9 packs. All other inputs stay the same.
Try this with your own numbersFormula and worked example
Packs = ceiling(net quantity × (1 + waste) / pack quantity)
| Input | Example |
|---|---|
| Net quantity needed | 86 |
| Waste allowance | 10 % |
| Units per supplier pack | 12 |
| Calculated result | 8 packs |
Choose reliable inputs
Plan materials, travel and crew time before committing to a price. For this roofing calculation, use a completed job sheet, supplier quotes and a crew schedule and a consistent definition of one roofing job.
- Materials: include the portion belonging to this roofing job or reporting period, and record where the value came from.
- Disposal: include the portion belonging to this roofing job or reporting period, and record where the value came from.
- Weather-related downtime: include the portion belonging to this roofing job or reporting period, and record where the value came from.
Use the same currency for every money input. Do not mix weekly costs with monthly revenue, or count the same expense in both an itemised cost and allocated overhead.
Should I round before or after the waste allowance?
Apply waste before rounding. The allowance adds to net need; it is not a percentage loss from purchased quantity.
What if this input changes?
If net quantity needed rises by 10%, this example changes by 1 packs. This shows the effect of one change; it does not predict what will happen.
Open the calculator, save your baseline, then change the input supported by the least reliable estimate in your records. Compare both scenarios before committing to a price or capacity promise.