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This article describes the formula syntax and usage of the ISOWEEKNUM function 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.


Copy the example data in the following table, and paste it in cell A1 of a new Excel worksheet. For formulas to show results, select them, press F2, and then press Enter. If you need to, you can adjust the column widths to see all the data.







Number of the week in the year that 3/9/2012 occurs, based on weeks beginning on the default, Monday (10).


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