WEEKNUM
Returns the week number of a specific date within the year.
WEEKNUM(serial_number, [return_type])Arguments
serial_numberThe datereturn_typeWeek start day (default Sunday)(optional)
=WEEKNUM(TODAY())Week number of today
=WEEKNUM(A1,2)Week starting Monday
=WEEKNUM("2024-12-31")Last week of year
- •Return type 1 = Sunday start (default)
- •Return type 2 = Monday start
- •Use ISOWEEKNUM for ISO standard
- •Assuming WEEKNUM always follows the ISO 8601 standard for week numbering - by default it doesn't, and the return_type argument needs to be set specifically (like 21) to match ISO week numbering conventions used in much of Europe
- •Not specifying return_type when the week is expected to start on a particular day, leading to a week number that's off by one compared to a company's or country's actual calendar convention
- •Confusing WEEKNUM with ISOWEEKNUM - ISOWEEKNUM always uses the ISO standard directly, while WEEKNUM requires the correct return_type argument to replicate that same behavior
ISOWEEKNUMAlways calculates the ISO 8601 week number directly, without needing a return_type argument like WEEKNUM.WEEKDAYReturns which day of the week a date falls on, rather than which week of the year it belongs to.DATEOften used to construct a specific date before feeding it into WEEKNUM for testing or reference.Why does WEEKNUM give a different week number than I expect?
By default, WEEKNUM assumes weeks start on Sunday, which differs from the ISO 8601 standard used in most of Europe (weeks starting Monday). Use return_type 21 to match ISO week numbering, or use ISOWEEKNUM directly.
What's the difference between WEEKNUM and ISOWEEKNUM?
ISOWEEKNUM always follows the ISO 8601 standard automatically. WEEKNUM is more flexible but requires the correct return_type argument to replicate that same ISO behavior.
Can WEEKNUM start weeks on a day other than Sunday or Monday?
Yes, various return_type values let you specify different starting days for the week - check Excel's documentation for the exact code matching your desired convention.
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