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Dates function

Excel ISOWEEKNUM function

ISOWEEKNUM returns the ISO 8601 week number of a date. ISO weeks start on Monday, and week 1 is the week that contains the year’s first Thursday. That means the first days of January can belong to the last week of the previous year, and late December to week 1 of the next.

=ISOWEEKNUM(date)

Arguments

date
The date to get the week number for.

ISOWEEKNUM examples

Every result below was calculated from this sample sheet by the same spreadsheet engine the Cellwise app uses.

AB
1ShipmentDate
2SH-11Dec 28, 2026
3SH-12Jan 1, 2027
4SH-13Jan 3, 2027
5SH-14Jan 4, 2027
6SH-15Dec 30, 2024

Download this sheet as an Excel practice file (.xlsx)

  1. =ISOWEEKNUM(B3)

    Result: 53

    January 1, 2027 is a Friday, so it belongs to ISO week 53 of 2026, not week 1.

  2. =ISOWEEKNUM(B5)

    Result: 1

    Monday January 4, 2027 starts ISO week 1 of 2027.

  3. =ISOWEEKNUM(B6)

    Result: 1

    It works the other way too: Monday December 30, 2024 is in ISO week 1 of 2025.

  4. =YEAR(B3-WEEKDAY(B3,2)+4)&"-W"&TEXT(ISOWEEKNUM(B3),"00")

    Result: 2026-W53

    A sortable ISO label: 2026-W53. The year comes from the Thursday of the same week, which is how ISO decides which year a week belongs to.

How to use ISOWEEKNUM

  1. Click the cell next to the date.
  2. Type =ISOWEEKNUM(B2) and press Enter.
  3. For labels or grouping, build the ISO year with YEAR(B2-WEEKDAY(B2,2)+4).

Common ISOWEEKNUM mistakes

Pairing the ISO week with YEAR(date)

YEAR(B3) says 2027 for January 1, 2027, but its ISO week is 53 of 2026. Use the Thursday-based ISO year shown above.

Confusing it with WEEKNUM

WEEKNUM starts week 1 on January 1 and weeks on Sunday by default. ISOWEEKNUM equals WEEKNUM(date,21), not WEEKNUM(date).

Older Excel versions

ISOWEEKNUM needs Excel 2013 or later. In Excel 2010 use WEEKNUM(date,21).

ISOWEEKNUM FAQ

What is an ISO week number?

A week numbering standard (ISO 8601) used widely in Europe and in many business systems: weeks run Monday to Sunday and week 1 contains the first Thursday of the year. A year has 52 or 53 ISO weeks.

Why is January 1 in week 52 or 53?

When January 1 falls on a Friday, Saturday or Sunday, that week has more days in the old year, so it counts as the old year’s last week.

Is ISOWEEKNUM the same as WEEKNUM with 21?

Yes, they return the same number.

Guides that use ISOWEEKNUM

Related functions

Updated