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WEEKNUM Function
Summary
The Excel WEEKNUM function extracts the week number from any given date, enabling effective date-based analysis, reporting, and timeline organization across various week numbering systems.
Syntax
WEEKNUM(serial_number,[return_type])Parameters
| Parameter | Type | Required | Description |
|---|---|---|---|
| serial_number | Date |
Yes | The date to analyze - use DATE function or cell reference |
| return_type | Number |
No | Week definition: 1=Sunday (default), 2=Monday, 11-17=other days, 21=ISO system |
Using the WEEKNUM Function
WEEKNUM transforms dates into their corresponding week numbers, perfect for weekly summaries, fiscal reporting, progress tracking, and grouping time-based data by week periods.
Common WEEKNUM Examples
Basic Week Number
=WEEKNUM(DATE(2023,3,9))Returns week 10 for March 9, 2023 using default Sunday start
Monday Week Start
=WEEKNUM(A2,2)Returns week 11 for date in A2 using Monday as first day
ISO 8601 Standard
=WEEKNUM(B2,21)Returns European week number using first Thursday system
Dynamic Current Week
=WEEKNUM(TODAY())Gets current week number using today's date
Frequently Asked Questions
Common Errors and Solutions
#NUM!
Cause: return_type outside valid range 1-21
Solution: Use values from 1,2,11-17, or 21 only
#VALUE!
Cause: serial_number not recognized as valid date
Solution: Use DATE function or ensure cell contains proper date
#NUM!
Cause: Date before Excel epoch (1900-01-01)
Solution: Verify date falls within valid Excel date range
Notes
- Excel stores dates as serial numbers starting January 1, 1900 = 1
- Maximum 53 weeks possible in leap years
- Use 21 for ISO 8601 compliance (Monday weeks)
- System 1 most common for US reporting
- Perfect companion for YEAR, MONTH functions
Compatibility
Available in: Excel 2007, Excel 2010, Excel 2013, Excel 2016, Excel 2019, Excel 2021, Microsoft 365
Not available in: Excel 2003, Excel XP, Excel 2000, Excel 97
Content last reviewed: December 11, 2025
Update frequency: As needed
Excel versions tested: Excel 2007+