Summary

The Excel ISOWEEKNUM function returns the ISO 8601 week number for a date. It always uses Monday as the first day of the week and follows the ISO rule that week 1 is the week containing the year's first Thursday.

This matters most in reporting, planning, and dashboards where week numbers must stay consistent across teams and countries. Because ISO weeks can cross calendar-year boundaries, a date near January 1 may belong to the last ISO week of the previous year.

Syntax

=ISOWEEKNUM(date)

ISOWEEKNUM has one required argument: date. You can pass a cell reference, a date serial number, or a formula that returns a valid Excel date. For reliable results, use real Excel dates or DATE(...) instead of ambiguous text dates when possible.

Arguments

• date - The date you want to evaluate. Excel expects a valid date value.

ISOWEEKNUM vs WEEKNUM

Both functions return week numbers, but they do not always follow the same rules.

If your workbook is supposed to follow ISO 8601, ISOWEEKNUM is usually the clearest choice. Microsoft also documents that WEEKNUM(...,21) uses the same ISO-based system.

Using ISOWEEKNUM

Think of ISOWEEKNUM as a week-labeling function. Give it a date and it tells you which ISO week that date belongs to. The important detail is that ISO weeks are not tied directly to January 1. Instead, ISO week 1 is defined by the first Thursday of the year, which keeps every ISO week aligned to a full Monday-to-Sunday structure.

That rule is why dates near New Year can look surprising at first. For example, January 1, 2023 falls in ISO week 52, not week 1, because it still belongs to the last ISO week of 2022. If you build year-week labels, this also means YEAR(date) is not always the true ISO year near the boundary.

For a true ISO year-week label, use a formula like this:

=YEAR(A1-WEEKDAY(A1,2)+4)&"-W"&TEXT(ISOWEEKNUM(A1),"00")

Shifting to the Thursday of the same ISO week makes the year part line up with the ISO calendar, not just the regular calendar year.

Example 1 - Basic ISO Week Number

Use ISOWEEKNUM to return the ISO week number for a standard date. This is the most direct use case: you already have a valid date and simply want the ISO week bucket it belongs to.

=ISOWEEKNUM("2026-04-15")   // Returns 16

Because April 15, 2026 falls well inside the year, there is no year-boundary complication here. That makes this a clean example of the function's normal behavior.

Example 2 - A Date at the Year Boundary

Dates near January 1 can belong to the previous ISO year. This is where ISO week numbering often differs from what users expect at first glance.

=ISOWEEKNUM("2023-01-01")   // Returns 52

January 1, 2023 is a Sunday, so it still falls in the last ISO week of 2022. If you group records by ISO week, this date should stay with the final reporting week of that ISO year rather than being treated as week 1.

Example 3 - A Date in ISO Week 1

When January begins in the same Monday-to-Thursday block as the first Thursday, the date belongs to week 1. This example shows the opposite of the previous case.

=ISOWEEKNUM("2024-01-01")   // Returns 1

Since January 1, 2024 is a Monday, the week containing it also contains the year's first Thursday. That is why Excel correctly labels it as ISO week 1.

Example 4 - Build a True ISO Year-Week Label

If you need labels like 2026-W16, calculate the ISO year first instead of using plain YEAR(date). This avoids incorrect labels for dates that sit at the start or end of a calendar year.

=YEAR(A1-WEEKDAY(A1,2)+4)&"-W"&TEXT(ISOWEEKNUM(A1),"00")

This pattern is useful when you want a single text label for charts, pivot tables, exports, or weekly summaries. It keeps the year and week aligned to the same ISO calendar system.

Conclusion Recap

• Summary: ISOWEEKNUM returns the ISO 8601 week number for a date.
• Week start: ISO weeks always begin on Monday.
• Week 1 rule: Week 1 is the week containing the first Thursday of the year.
• Boundary behavior: Dates near January 1 can belong to the previous or next ISO year.
• Alternative: WEEKNUM(date,21) follows the same ISO-based system.
• Best practice: Use an ISO-year formula when building year-week labels for reporting.