Summary

The Excel WEEKNUM function returns the week number of a date within the year.

WEEKNUM is useful for weekly reporting, scheduling, production cycles, sprint planning, and any workbook that groups records by week instead of by day or month.

Syntax

=WEEKNUM(serial_number, [return_type])

serial_number is the date you want to evaluate. The optional return_type changes the week system and the starting day of the week.

Arguments

• serial_number - A valid Excel date.
• return_type - [optional] A number that determines how weeks are counted.

WEEKNUM Systems

Microsoft documents two main systems for this function.

If you need strict ISO week numbering, WEEKNUM(date,21) follows the same ISO-based system documented by Microsoft. If you want a simpler Sunday-start or Monday-start week number, use the standard return types instead.

Using WEEKNUM

WEEKNUM is most useful when your reporting is organized by week. For example, a sales log, manufacturing log, or sprint tracker can use a helper column with WEEKNUM so records can be grouped and filtered more easily.

Return type 2 is often preferred in business models because it starts the week on Monday. Return type 21 is for ISO week numbering, where week 1 is defined by the first Thursday of the year rather than simply by January 1.

Because week systems differ, the same date can return different week numbers depending on the return type. That is the most important detail to keep in mind when using WEEKNUM in shared reports.

Example 1 - Return the Current Week Number

This is the most basic use of WEEKNUM: return the week number for today.

=WEEKNUM(TODAY())

This is helpful for live dashboards, weekly summaries, and current-week reporting.

Example 2 - Use Monday as the Week Start

Return type 2 starts the week on Monday instead of Sunday.

=WEEKNUM(B2,2)

This is often easier to align with business calendars, especially when teams treat Monday as the first workday of the week.

Example 3 - Use ISO Week Numbering

Return type 21 switches the function to the ISO 8601 system.

=WEEKNUM("2026-01-01",21)

This is useful when your reports need to match international or ISO-based week numbering rules.

Example 4 - Build a Week Label

You can combine WEEKNUM with text to create display labels.

="Week " & WEEKNUM(B2)

This pattern works well for chart labels, sprint names, and weekly headings in reports.

Conclusion Recap

• Summary: WEEKNUM returns the week number for a date.
• Default behavior: Standard week numbering uses the week containing January 1 as week 1.
• Business option: Return type 2 starts weeks on Monday.
• ISO option: Return type 21 uses the ISO 8601 system.
• Common uses: Weekly grouping, sprint planning, and recurring reports.
• Important detail: The same date can return different week numbers under different systems.