How to use Excel ISOWEEKNUM Function


ISOWEEKNUM function in Excel is a type of DATE/TIME function. This function is used to find week number in ISO standard form (1-54). In ISO system, a week start on Monday and 1st week of the year always have a Thursday. For example, JAN 14 2018 gives 2 as the result when used in ISOWEEKNUM function.




  • Serial_Date- This is a Date in a valid excel format.


  • In General formatting of Excel, Dates are in form of serial numbers. Serial number 1 is for 1 JAN, 1900 and the number increases from thereon. For getting it in readable form use the formatting tab.
  • The argument used must represent a valid date in Excel format.
  • If the arguments used have number values which are out of range of valid excel date, #NUM error is displayed
  • If the arguments used have strings which cannot be converted into valid dates, #VALUE error is displayed.


In this example, Date in form of text and serial number are given in column A and column B respectively. ISOWEEKNUM function is used in column D to retrieve the week number in ISO standards of the given date.

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