Returns the number of the ISO (International Organization for Standardization) week of the year where the provided date falls.

Sample Usage

ISOWEEKNUM(DATE(1969, 7, 20))




  • date - The date for which to determine the ISO week number. Must refer to a cell containing a date, number, or function returning a date type.


  • When entering the date, use the DATE function, as text values might return errors.
  • Per the European system for numbering weeks (also known as ISO 8601):
    • Weeks begin on Monday and end on Sunday.
    • Week 1 of the year is the week containing the first Thursday of the year.
      • As a result, if the first Thursday is after January 4, at least one of the following days, January 1-3, will have an ISOWEEKNUM of 52 or 53.
      • Similarly, if the first Thursday is before January 4, at least one of the following days, December 29-31, will have an ISOWEEKNUM of 1. 
      • If the first Thursday is January 4, December 29-31 of the previous year will have ISOWEEKNUM of 52 and January 1-7 of the current year will have ISOWEEKNUM of 1.

See Also

  • WEEKNUM: Returns a number representing the week of the year where the provided date falls.
  • WEEKDAY: Returns a number representing the day of the week of the date provided.
  • DAYS360: Returns the difference between two days based on the 360-day year used in some financial interest calculations.
  • DATE: Converts a year, month, and day into a date.
  • DATEVALUE: Converts a provided date string in a known format to a date value.
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