Scrap rate starts calculator finds how many parts to start a job with, given a desired quantity of good parts and an expected scrap rate, so a run doesn’t come up short.
Work out how many parts to start in order to end up with the number of good parts you actually need, given an expected scrap rate.
Parts to start = desired good parts / (1 – scrap rate). Expected scrap quantity = parts to start – desired good parts.
How it works
If a process loses a known percentage of parts to scrap, starting exactly the number of good parts you need will leave you short. Inflating the start quantity by the scrap rate covers the expected loss so the finished good count lands where you planned.
FAQ
Should I round the result up? Yes, always round the parts-to-start number up to a whole part, since a fractional part cannot be started or finished.
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How it works
If scrap is expected at a known rate, starting with exactly the desired quantity will almost always deliver fewer good parts than needed. Dividing the desired good-parts quantity by one minus the scrap rate inflates the start quantity to compensate; it’s a standard job-quoting and material-ordering calculation, especially for processes with a known average scrap percentage.
FAQ
Where does my expected scrap rate come from? Ideally from historical data on the same or a similar part, process, and material combination; without history, a conservative estimate based on part complexity and process capability is a reasonable starting point, refined once real run data is available.
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