The ISOWEEKNUM function returns the ISO week number of a specific date.



date The date for which you want to determine the ISO week number.


When dealing with dates in Excel and needing to identify the week number according to the ISO 8601 standard, ISOWEEKNUM comes to the rescue. This function provides a straightforward way to extract the ISO week number corresponding to a given date, making it particularly handy for organizing and categorizing dates based on the standardized week numbering system used in business and international contexts. The ISO week numbering system assigns each week a number from 1 to 53, with the first week of the year containing the first Thursday being week 1.


Suppose you have a date cell containing '05/15/2022'. To determine the ISO week number for this date, you would use the formula: =ISOWEEKNUM(A1). If A1 contains the date '05/15/2022', the ISOWEEKNUM function will return '19', indicating that '05/15/2022' falls in the 19th ISO week of the year.


What is the significance of the ISO week numbering system?

The ISO week numbering system aims to provide a standardized way of identifying weeks within a year, with week 1 defined as the week that contains the first Thursday of the year. This system helps facilitate international communication and consistent week-based calculations across different regions.

How does ISOWEEKNUM differ from the WEEKNUM function in Excel?

While the WEEKNUM function in Excel calculates week numbers based on the user-defined start of the week, ISOWEEKNUM strictly adheres to the ISO 8601 standard for week numbering. ISOWEEKNUM ensures consistent week numbering across different geographies and cultures, making it ideal for international business and compliance purposes.

Can the ISOWEEKNUM function return a week number greater than 52?

Yes, the ISO week numbering system used by ISOWEEKNUM allows for week numbers to reach up to 53 in a year, ensuring alignment with the standard's definition of weeks. This flexibility accommodates cases where the first day of the year falls on a Thursday or a leap year scenario.

Is the ISO week number output by ISOWEEKNUM dependent on the regional settings of Excel?

No, the ISO week number returned by the ISOWEEKNUM function is not affected by the regional settings in Excel. It strictly follows the ISO 8601 standard for week numbering, ensuring consistent results regardless of the user's location settings.

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