ISOWEEKNUM
Returns the ISO 8601 week number of a given date. Under the ISO standard, weeks start on Monday and week 1 is the week containing the first Thursday of the year.
ISOWEEKNUM(date)Arguments
dateThe date for which you want the ISO week number.
=ISOWEEKNUM("2026-01-05")Monday Jan 5, 2026 is in ISO week 2
=ISOWEEKNUM("2026-01-01")Thursday Jan 1, 2026 belongs to ISO week 1
=ISOWEEKNUM(TODAY())ISO week number of today
- •ISO weeks always start on Monday and end on Sunday
- •An ISO week may belong to the previous or next calendar year (Dec 31 can be in week 1 of the next year)
- •Use WEEKNUM if you need a non-ISO numbering system
- •Available since Excel 2013
- •Confusing ISOWEEKNUM with WEEKNUM's default behavior - WEEKNUM assumes weeks start on Sunday unless you specify an ISO-matching return_type, while ISOWEEKNUM always follows the Monday-start ISO 8601 standard automatically
- •Forgetting that the very first days of January can belong to the previous year's final ISO week, or the last days of December can belong to the next year's ISO week 1, which surprises anyone expecting the week number to reset cleanly at the calendar year boundary
- •Passing a date as unrecognized text instead of a genuine Excel date value, which produces an error rather than the expected week number
WEEKNUMOffers configurable week-numbering conventions, only matching ISO 8601 behavior when the return_type argument is explicitly set to 21.WEEKDAYReturns which day of the week a date falls on, rather than which week of the year it belongs to.YEARExtracts a date's calendar year, which can differ from the year implied by its ISO week near the start or end of December/January.How is ISOWEEKNUM different from WEEKNUM?
ISOWEEKNUM always follows the ISO 8601 standard, with weeks starting Monday and week 1 defined as the week containing the year's first Thursday. WEEKNUM defaults to a Sunday-based system unless you explicitly set return_type to 21 to match ISO behavior.
Why does ISOWEEKNUM return a week number that seems to belong to the wrong year?
ISO weeks don't align perfectly with calendar years - late December dates can fall in week 1 of the following year, and early January dates can fall in the last week of the previous year, depending on which day of the week the year starts on.
Do ISO weeks always start on the same day?
Yes, ISO 8601 weeks always start on Monday and end on Sunday, unlike some other week-numbering conventions that start on Sunday or Saturday.
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