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.
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.
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