This article describes the formula syntax and usage of the ISOWEEKNUM function (function: A prewritten formula that takes a value or values, performs an operation, and returns a value or values. Use functions to simplify and shorten formulas on a worksheet, especially those that perform lengthy or complex calculations.) in Microsoft Excel.


Returns number of the ISO week number of the year for a given date.



The ISOWEEKNUM function syntax has the following arguments.

  • Date    Required. Date is the date-time code used by Excel for date and time calculation.


  • Microsoft Excel stores dates as sequential numbers so they can be used in calculations. By default, January 1, 1900 is serial number 1, and January 1, 2008 is serial number 39448 because it is 39,448 days after January 1, 1900.
  • If the date argument is not a valid number, ISOWEEKNUM returns the #NUM! error value.
  • If the date argument is not a valid date type, ISOWEEKNUM returns the #VALUE! error value.


The workbook below shows examples of this function. Inspect them, change existing formulas, or enter your own formulas to learn how the function works.

To work more in-depth with the example data in Excel, download the embedded workbook to your computer, and then open it in Excel.

Top of Page Top of Page

Applies to:
Excel 2013, Excel Online