Related Function:

The ISOWEEKNUM function returns the ISO week number of a supplied date.

  • This function was only introduced in Excel 2013 and so is not available in earlier versions of Excel.




Argument Description
date The date argument

Note: It is recommended that the supplied date argument is input as either:

  • A reference to a cell containing a date, or
  • A date returned from a formula

Warning: If you attempt to input the date as text, there is a chance that Excel may misinterpret this due to different date systems, or date interpretation settings.


  A B C
1 Date Formula Result
2 Fri 01-Jan-2015 =ISOWEEKNUM(A1) 1
3 Fri 02-Feb-2015 =ISOWEEKNUM(A2) 6
4  Sun 10-May-2015 =ISOWEEKNUM(A3) 19

Common Function Error(s)

Problem What went wrong
#VALUE! Occurs if the supplied date argument is a text value that can not be interpreted as a date
#NUM! Occurs if the supplied date argument is a numeric value, but is not recognized as a valid date

The ISO (International Organization for Standardization) week date system is a leap week calendar system that is part of the ISO 8601 date and time standard.

Each week of the ISO date system starts on a Monday, with week number 1 set to the first week of the year that contains a Thursday.