The ISOWEEKNUM function in Excel is a valuable tool for determining the week number of any given date based on the ISO 8601 standard. This standard defines the first week of the year as the one that contains the first Thursday of the year, which is particularly useful for businesses and organizations that operate on a week-based calendar system.

Syntax

ISOWEEKNUM(serial_number)

• serial_number: The date for which you want to calculate the ISO week number. This can be a date format or a reference to a cell containing a date.

Example #1

ISOWEEKNUM("2023-01-04")

Using this function, the ISO week number for January 4, 2023, is calculated. The result is 1 because this date falls within the first week of the year.

Example #2

ISOWEEKNUM("2023-12-31")

This function retrieves the ISO week number for December 31, 2023. The outcome is 52, as it is the last week of the year.

Example #3

ISOWEEKNUM(A1)

If cell A1 contains the date “2023-07-04”, the function calculates the ISO week number for this date, resulting in 27, which indicates the week number of this summer holiday.

Error handling

• VALUE! – This error occurs if the provided date is not in a recognizable date format.
• NUM! – This error indicates that the date provided is invalid, like a date outside of the Excel date range.

Conclusion

The ISOWEEKNUM function is an essential tool in Excel for users needing to track and analyze date-related information in relation to ISO week numbers. Its straightforward syntax allows for easy implementation, making it an efficient choice for users in various industries requiring accurate week-based data management.