The Excel ISOWEEKNUM Function

ISO Week Number

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.

A detailed description is provided on the Wikipedia ISO week date page.
Related Function:

Function Description

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

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

The syntax of the Isoweeknum function is:


where the date argument is the Excel date that you want to find the ISO week number of.

Note that it is advised that you do not type dates directly into functions, as Excel may interpret text representations of dates differently, depending on the date interpretation settings on your computer. Therefore the date argument for the Isoweeknum function should be input as either:


ISOWEEKNUM Function Examples

  A B
1 01/01/2015 =ISOWEEKNUM( A1 )
2 02/02/2015 =ISOWEEKNUM( A2 )
  A B
1 01/01/2015 1
2 02/02/2015 6

For further information and examples of the Excel Isoweeknum function, see the Microsoft Office website.

ISOWEEKNUM Function Errors

Common Errors
#NUM! - Occurs if the supplied date argument is a numeric value, but is not a valid date.
#VALUE! - Occurs if the supplied date argument is a text value that can not be interpreted as a date.