I am not sure that there is one single formula or universal method of determining scrap. Depending upon processes you might be able to use SPC & process capability studies to predict scrap. Other means would involve looking at historical data on yield. Scrap can unfortunately be produced in many ways. Depending on your process, some or all of these may apply.
Scrap caused by non-conforming part itself (not reworkable). Setup Error/Materials Conformance/Process error.
Scrap caused by mis-handling.
Scrap caused by incorporating a good component into what turns out to be a non-conforming assembly (not reworkable).
Scrap caused by obsolescence due to an Engineering change.
This may be a start for you. Others posters will have additional information. Are you looking for generic information or could you be a bit more detailed on your query?
%Scrap = ((input-output)/input)* 100.
This following comment may sound a bit flippant but the reason only matters if one is looking to change the percentage. Some companies and some managers may have a vested interest in not assigning reasons.
