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ISOWEEKNUM Function
Summary
The ISOWEEKNUM function returns the ISO week number of the year for a specified date, following the ISO 8601 standard where weeks start on Monday and the first week of the year contains at least 4 days.
Syntax
ISOWEEKNUM(date)Parameters
| Parameter | Type | Required | Description |
|---|---|---|---|
| date | Date |
Yes | A valid Excel date or reference to a cell containing a date |
Using the ISOWEEKNUM Function
ISOWEEKNUM provides standardized week numbering essential for international business reporting, financial periods, and data analysis that spans multiple countries. Unlike Excel's WEEKNUM function, ISOWEEKNUM always uses Monday as the first day of the week and follows ISO 8601 rules.
Common ISOWEEKNUM Examples
Basic ISO Week Calculation
=ISOWEEKNUM(DATE(2023,3,9))Returns 10, confirming March 9, 2023 falls in ISO week 10.
Week Number from Cell Reference
=ISOWEEKNUM(A2)Returns the ISO week number for the date in cell A2.
Combine with YEAR and MONTH
=YEAR(A2)&"-W"&TEXT(ISOWEEKNUM(A2),"00")Creates standard ISO week format like '2023-W10'.
Frequently Asked Questions
Common Errors and Solutions
#VALUE!
Cause: Date argument is not a valid date
Solution: Ensure cell contains proper date format or use DATE() function
#NUM!
Cause: Date argument is not a valid number
Solution: Check that referenced cells contain numeric date values
#NAME?
Cause: Function misspelled
Solution: Use exact spelling: ISOWEEKNUM
Notes
- ISO weeks: 1-53 (rarely 53 weeks in a year)
- January 1st may be in previous year's week 52/53
- December 31st may be in next year's week 1
- Excel stores dates as serial numbers starting from 1900-01-01
Compatibility
Available in: Excel 2013, Excel 2016, Excel 2019, Excel 2021, Excel 365
Not available in: Excel 2010, Excel 2007, Excel 2003
Content last reviewed: December 9, 2025
Update frequency: As needed
Excel versions tested: Excel 2013+