UDF iso Week Number

Easily identify the iso weeknumber with a formula.

Paste this UDF into a module in the VB Editor (ALT+F11) then use the following formula on a worksheet.


This will return the iso week number that cell A1 falls under – ie 01/03/2017 = week 9

Public Function IsoWeekNumber(d1 As Date) As Integer
Dim d2 As Long
d2 = DateSerial(Year(d1 - Weekday(d1 - 1) + 4), 1, 3)
IsoWeekNumber = Int((d1 - d2 + Weekday(d2) + 5) / 7)
End Function

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