How to use this calculator
- Enter or choose good units required.
- Enter or choose expected scrap (share of units started).
- Enter or choose material per unit.
- Enter or choose material already in stock.
- The units to start updates as you type. Use Copy result or Share to keep it.
Method
Starts = ⌈Good ÷ (1 − Scrap%)⌉; Gross material = Starts × Material per unit; Net = Gross − Stock
- Scrap%
- share of units started that end up scrapped
Worked example
- Starts = 10,000 ÷ 0.96 = 10,416.7, rounded up to 10,417.
- Material = 10,417 × 0.25 = 2604.25.
- To order = 2604.25 − 500 = 2104.25.
To finish 10,000 good units at 4% scrap, start 10,417 and order 2104.25 units of material.
What the result means
Dividing by (1 − scrap) is not the same as adding the scrap percentage: adding 4% would give 10,400 starts and leave you short.
Assumptions and limits
- Scrap is a share of units started, not of good units.
- Every unit uses the same material, whether it ends up good or scrap.
- For multi-step processes, apply each step’s scrap rate in turn, starting from the last step.
Questions
Why not just add 4%?
Because the 4% is lost from the larger starting quantity. 10,400 × 0.96 = 9984, short of 10,000.
What if my scrap rate is quoted on good output?
Then multiply good units by (1 + rate) instead.