The Excel ISOWEEKNUM function takes a date and returns a week number (1-54) that follows ISO standards, where weeks begin on Monday and week number 1 is assigned to the first week in a year that contains a Thursday.


Get ISO week number for a given date

Return value 

A number between 1 and 54.


  • date - A valid Excel date in serial number format.

How to use 

The ISOWEEKNUM function returns a week number based on ISO standards. Under this standard, weeks begin on Monday and the week number 1 is assigned to the first week in a year that contains a Thursday, following ISO 8601.

ISOWEEKNUM takes just one argument, date, which must be a valid Excel date.


In the example shown, the formula in D5, copied down, is:

=WEEKNUM(B5) // default week number

The formula in E5, copied down the table, is:

=ISOWEEKNUM(C5) // ISO week number

By default the standard WEEKNUM function will start week number 1 on the first day of the year, then increment week numbers on Sundays after that. The ISOWEEKNUM function increments on Mondays, and starts week 1 on the first week that contains a Thursday.

The WEEKNUM function can also be configured to output an ISO week number, by setting the return_type argument to 21. The formula below will output the same week numbers seen in column E of the example:

=WEEKNUM(B5,21) // ISO week number


  • ISOWEEKNUM returns #VALUE! if date is not recognized as a valid date.
  • ISOWEEKNUM returns #NUM! if date is out of range.
Dave Bruns Profile Picture

AuthorMicrosoft Most Valuable Professional Award

Dave Bruns

Hi - I'm Dave Bruns, and I run Exceljet with my wife, Lisa. Our goal is to help you work faster in Excel. We create short videos, and clear examples of formulas, functions, pivot tables, conditional formatting, and charts.