Return the ISO Week Number in Excel with this UDF. This is a simple to use UDF (user defined function) that returns the number of the week in the year that a specific date occurs.
Function ISOWEEKNUMBER(InDate As Date) As Long
Dim D As Date
D = DateSerial(Year(InDate - WeekDay(InDate - 1) + 4), 1, 3)
ISOWEEKNUMBER = Int((InDate - D + WeekDay(D) + 5) / 7)
End Function