Excel ISOWEEKNUM Function

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.