Using the Excel WEEKNUM Function
SummaryThe WEEKNUM function will return the number of a week when provided a date. The function begins each week on Sunday and the first week of the year is considered the week that contains January 1st. If you would like to adhere to a European week numbering system you can use the ISOWEEKNUM function. ISOWEEKNUM considers the week that contains the first Thursday of the year to be week 1.
Syntax=WEEKNUM (serial_number, [return_type])
The serial_number argument is required and will contain the date where you would like to find the week number. It is recommended to enter dates using the DATE function to avoid any errors that may arise from entering dates as text. Dates can also be entered as the output of other functions or formulas.
Return_type is an optional field that determines which day of the week should be considered the first day. By default, this field is set to 1.
Usage NotesISO 8601 Standards
Using return_type 21 OR the ISOWEEKNUM function will cause the first week of the year to be that week which contains the first Thursday. This follows ISO 8601 standards.
The function will return the #VALUE! error if serial_date is out of the range of valid date values or the #NUM! error if return_type is a value not specified in the above table.