MS Excel: WEEKNUM function to get the week number from any date

The WEEKNUM function in Microsoft Excel returns the week number of a given date. This is especially useful for weekly reporting, payroll, inventory planning, or project management where you want to organize or group data by calendar weeks.

Whether you need to calculate which week a date falls into or sort reports by weekly intervals, WEEKNUM helps you do it efficiently.

Syntax

=WEEKNUM(serial_number, [return_type])

Arguments:

Argument	Required	Description
serial_number	✅ Yes	A valid Excel date (can be a date, cell reference, or formula).
return_type	❌ No	A number (1 or 2, or 11–21) indicating which day starts the week. Default is 1 (week starts on Sunday).

Return Type Options

Return Type	Week Starts On	Notes
1 (default)	Sunday	Week 1 starts on Jan 1
2	Monday	Week 1 starts on Jan 1
11–17	Mon–Sun	ISO-like variations
21	Monday	ISO 8601 standard (use ISOWEEKNUM)

Examples

Example 1: Week Number of a Date

=WEEKNUM(DATE(2025,5,2))

Result: 18(May 2, 2025 falls in the 18th week of the year if weeks start on Sunday.)

Example 2: Week Number Starting from Monday

=WEEKNUM("02/05/2025", 2)

Result: 18(Using Monday as the first day of the week.)

Example 3: Use with TODAY() for Dynamic Week Tracking

=WEEKNUM(TODAY(), 2)

Returns the current week number based on today’s date and a Monday start.

Sample Table

Date	Formula	Return Type	Result	Description
01/01/2025	=WEEKNUM(A2, 1)	1	1	First week of the year
15/01/2025	=WEEKNUM(A3, 2)	2	3	Third week, Monday start
31/12/2025	=WEEKNUM(A4)	1	53	Last week of the year

Notes

• The WEEKNUM function starts counting from January 1st, regardless of what day of the week that falls on.

• For ISO 8601 week numbering (where Week 1 is the week containing the first Thursday of the year), use ISOWEEKNUM instead.

• Excel stores dates as serial numbers; invalid inputs will result in a #VALUE! error.

Related Functions

Function	Description
ISOWEEKNUM()	Returns ISO-compliant week number
WEEKDAY()	Returns the day of the week as a number
TODAY()	Returns the current date
TEXT()	Format a date to show full names

Summary Table

Feature	Description
Function Name	WEEKNUM
Purpose	Get week number of a given date
Customizable Week Start	✅ Yes (via return_type)
Date System	Works with Excel 1900/1904 systems
Introduced In	Excel 2007+

Conclusion

The WEEKNUM function simplifies weekly data organization in Excel. Whether you’re tracking deadlines, planning shifts, or preparing weekly reports, this function ensures accurate and dynamic week number calculations.