You have to tell excel where to find it:

=personal.xlsb!ROP(100,.5,.5)

Another option would be to save your personal workbook as an addin. Then excel
will find it (if you continue to have excel open it when excel starts).

Michael Smith wrote:

Thanks Dave, below is my corrected UDF, it's saved in Module1 in modules
folder under VBAproject(PERSONAL.XLSB)
I then go into any open workbook and type into a cell
=ROP(100,.5,.5)

Function ROP(Usage, LeadTime, SafetyStock)
Value = (Usage / 12 * LeadTime) + (Usage / 12 * SafetyStock)
ROP = Round(Value - (CLng(Value) < Value))
End Function

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Dave Peterson