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Date function

ISOWEEKNUM

Returns the ISO week number, where weeks start on Monday.

OccasionalDifficulty 1100 · ProficientUsage rank #183 of 520
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When to use it

The ISO 8601 week number: weeks start Monday and week 1 is the one containing the first Thursday of the year. Standard in Europe and logistics.

The shape of it

Syntax
=ISOWEEKNUM(date)

Worked examples

  • ISO week

    =ISOWEEKNUM(DATE(2025,3,15)) → 11

    Mid-March.

  • New year

    =ISOWEEKNUM(DATE(2025,1,1)) → 1

    January 1, 2025 is a Wednesday, inside week 1.

  • A year-end edge case

    =ISOWEEKNUM(DATE(2027,1,1)) → 53

    January 1, 2027 is a Friday and belongs to the last week of 2026.

Worth knowing

  • ISO year can differ from calendar year at the edges; compute it as YEAR(date+3-WEEKDAY(date,2)).
  • Group by ISO year and week together, not week alone.
  • Same as WEEKNUM(date,21).

Where it goes wrong

  • Late December and early January dates can carry the neighboring year's week number.
  • Text dates return #VALUE!.

Related

TODAYDATEMONTHYEARNOWDAY

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