Excel ISOWEEKNUM Function
The Excel ISOWEEKNUM function takes a date and returns a week number (1-54) that follows ISO standards, where weeks begin on Monday and week number 1 is assigned to the first week in a year that contains a Thursday.
- date - A valid Excel date in serial number format.
The ISOWEEKNUM function returns a week number based on ISO standards. Under this standard, weeks begin on Monday and the week number 1 is assigned to the first week in a year that contains a Thursday, following ISO 8601.
ISOWEEKNUM takes just one argument, date, which must be a valid Excel date.
Examples
In the example shown, the formula in D5, copied down, is:
=WEEKNUM(B5) // default week number
The formula in E5, copied down the table, is:
=ISOWEEKNUM(C5) // ISO week number
By default the standard WEEKNUM function will start week number 1 on the first day of the year, then increment week numbers on Sundays after that. The ISOWEEKNUM function increments on Mondays, and starts week 1 on the first week that contains a Thursday.
The WEEKNUM function can also be configured to output an ISO week number, by setting the return_type argument to 21. The formula below will output the same week numbers seen in column E of the example:
=WEEKNUM(B5,21) // ISO week number
Notes
- ISOWEEKNUM returns #VALUE! if date is not recognized as a valid date.
- ISOWEEKNUM returns #NUM! if date is out of range.
ISOWEEKNUM formula examples
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