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Date function
WEEKNUM
Returns the week number of the year.
CommonDifficulty 1050 · CapableUsage rank #77 of 520
When to use it
The week number of the year. The default system starts week 1 on January 1; type 21 gives ISO weeks where week 1 contains the first Thursday.
The shape of it
- Syntax
=WEEKNUM(serial_number, [return_type])
Worked examples
Default week
=WEEKNUM(DATE(2025,3,15)) → 11
Default system.
ISO week
=WEEKNUM(DATE(2025,3,15),21) → 11
ISO week, the same here but not always.
January 1
=WEEKNUM(DATE(2025,1,1)) → 1
January 1 is always week 1 in the default system.
Worth knowing
- Use ISOWEEKNUM or type 21 when reports must match European or supply-chain calendars.
- Week-ending date: date+7-WEEKDAY(date).
- Weeks split across years differ between systems; pick one and document it.
Where it goes wrong
- Default weeks can have 53 and week 1 can be a few days long.
- Text dates return #VALUE!.
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