Using the Excel ISOWEEKNUM Function
SummaryThe ISOWEEKNUM function will return the number of the week, a value from 1-54, when provided a date. The function follows ISO standards so all weeks begin on Monday and the first week of the year is considered the first week that contains a Thursday.
Date is a required value and contains the date of the week that you would like to find the number for. If dates are entered as text you may encounter errors. To avoid this, enter dates using the DATE function or use the output of other functions or formulas that are in serial number format.
Usage NotesSpecify First Day of the Week for the Year
If you require some flexibility and would like to specify what day will be considered the first day of the week for the year you can use the WEEKNUM function.