Return the ISO week number (1–53) for a date.
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=ISOWEEKNUM(A2)ISOWEEKNUM follows the ISO 8601 standard where weeks start on Monday and week 1 contains the first Thursday. Use WEEKNUM if you need a different week-start convention.
=ISOWEEKNUM(DATE(2026,1,5))→2Jan 5, 2026 is in ISO week 2.
=ISOWEEKNUM(DATE(2026,10,7))→417 October 2026 falls in ISO week 41.
=WEEKNUM(DATE(2026,1,1))→1WEEKNUM always puts 1 January in week 1.
Custom week start
=WEEKNUM(A2, 2)Code 2 starts weeks on Monday (non-ISO).
WEEKNUM counts week 1 as the week containing Jan 1 and defaults to Sunday start, so early-January results can differ.
Use =YEAR(A2)&"-W"&TEXT(ISOWEEKNUM(A2), "00"). Near New Year the ISO year can differ from the calendar year.
In the ISO system week 1 is the week containing the first Thursday, so the first days of January can belong to the last week of the previous year.
Use =DATE(A2, 1, 4)-WEEKDAY(DATE(A2, 1, 4), 2)+1+7*(B2-1), where A2 is the year and B2 the ISO week.
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