Excel has had a week number function for many versions, but Excel 2013 added a new week number function that complies with the ISO 8601 standard week number.
The existing week number function is WEEKNUM and can be used as below, assuming cell A1 has a date
The default week numbering starts on a Sunday, but there is an option to start the week on a Monday as follows
The new function is ISOWEEKNUM and can be used as below
The ISO standard week begins on the week with the year’s first Thursday.