Excel ISOWEEKNUM function


The Excel ISOWEEKNUM function calculates the ISO week number of the year for a given date. According to ISO standards, weeks start on Monday, and the first week of the year (week 1) is the week containing the first Thursday of the year. This function is particularly useful for planning and scheduling based on standardized week numbers.
  • date: The date for which you want to find the ISO week number. This must be a valid Excel date.
Return value
The ISO week number, ranging from 1 to 54.

How to use

Use ISOWEEKNUM when you need to determine the ISO week number for a specific date. This function is especially useful in professional settings where planning and reporting are done on a weekly basis following the ISO standard.



Basic ISO Week Number: To find the ISO week number of a specific date:

This formula returns the ISO week number for November 15, 2020.
Current ISO Week Number: To find the ISO week number for today’s date:
This formula calculates the current ISO week number based on today’s date.

Additional Notes

  • The ISOWEEKNUM function is particularly relevant for business and project management where ISO week numbering is a standard.
  • Ensure the date provided to ISOWEEKNUM is in a format recognized as a valid date in Excel; otherwise, it will return a #VALUE! error.
  • This function is ideal for reporting and scheduling tasks in a format that aligns with international standards.

Related Functions

Excel WEEKNUM function

The Excel WEEKNUM function calculates the week number of a date within the year, aiding in weekly planning and analysis.

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