Explanation
The WEEKNUM function takes a date and returns a week number (1-54) that corresponds to the week of year. The WEEKNUM function starts counting with the week that contains January 1. WEEKNUM takes two arguments: a date, and (optionally) return_type, which controls the scheme used to calculate the week number.
By default, the WEEKNUM function uses a scheme where week 1 begins on January 1, and week 2 begins on the next Sunday (when the return_type argument is omitted, or supplied as 1). With a return_type of 2, week 1 begins on January 1, and week 2 begins on the next Monday. See the WEEKNUM page for more information.
ISO week number
ISO week numbers, start on the Monday of the first week in a year with a Thursday. This means that the first day of the year for ISO weeks is always a Monday in the period between Jan 29 and Jan 4. Starting with Excel 2010, you can generate an ISO week number using 21 as the return_type:
=WEEKNUM(date,21)
Starting in Excel 2013, there is a new function called ISOWEEKNUM.
For more details, see Ron de Bruin's nice write-up on Excel week numbers.
Related formulas
Get day name from date
Get month from date
Get year from date
Related functions
WEEKNUM Function
The Excel WEEKNUM function takes a date and returns a week number (1-54) that corresponds to the week of year. The WEEKNUM function starts counting on the week that contains January 1. By default, weeks begin on Sunday, but this can be changed.
ISOWEEKNUM Function
The Excel ISOWEEKNUM function takes a date and returns a week number (1-54) that follows ISO standards, where weeks begin on Monday and week number 1 is assigned to the first week in a year that contains a Thursday.
