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Excel ISOWEEKNUM function

The ISO (International Organization for Standardization) week date system is a leap week calendar system, each week begins on Monday, and the week number 1 is set to the first week of the year that contains a Thursday. It is used by bookkeepers, accountants, human resources primarily as so on. In Excel, there is ISOWEEKNUM function, with it, you can quickly get the ISO week number from the given date.


The syntax for the ISOWEEKNUM function in Excel is:



  • Date: The Excel date that you want to find the ISO week number.


Return ISO week number between 1 and 54 based on a given date.


Example 1: Calculate the ISO week number based on specific dates

To calculate the ISO week number from given dates, please use the below formula, and all the week numbers of the ISO standard have been returned at once, see screenshot:


doc isoweeknum 1

Note: In the above formula, A2 is the cell contains the date that you want to calculate the week number from.

Example 2: Calculate the ISO week number of today

If you need to get the ISO week number based on current date, please apply the following formula. As today is 5/17/2019, ISOWEEKNUM will return 20 as following screenshot shown:


doc isoweeknum 2


1. This ISOWEEKNUM is only applied to Excel 2013 and later versions, in earlier versions of Excel, for returning a week number, you can only use the WEEKNUM function.

2. You can also replace the cell reference in the above formula with the date text string, such as:


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