Returns the ISO 8601 week number of the year for a given date.
This function works the same in Google Sheets as it does in Excel — same syntax and arguments.
ISOWEEKNUM(date)The ISOWEEKNUM function calculates the week number based on the ISO 8601 standard, where the week starts on Monday and the first week of the year contains the first Thursday. This function is useful for international business reporting where a consistent weekly calendar is required.
=ISOWEEKNUM("2023-01-05")Result: 1
Returns 1 because January 5, 2023, falls in the first ISO week of the year.
ISOWEEKNUM strictly follows the ISO 8601 standard, while WEEKNUM allows for different return types to define which day the week starts on.