The Excel ISOWEEKNUM function returns the ISO-8601 week number — weeks start Monday and week 1 is the week containing the year’s first Thursday. It’s the standard for international business, manufacturing, and project schedules.
Syntax
| Argument | Description | |
|---|---|---|
date | Required | The date to evaluate. |
How to use it
The crucial difference from WEEKNUM shows up at year boundaries: January 1, 2027 falls in ISO week 53 of 2026, because that week has more days in the old year. If your organization uses “week 1” consistently across countries, ISOWEEKNUM is the one.
Try it: interactive demo
Adjust the input and watch the formula and result update.
Tips & gotchas
ISO-8601 week number (Monday start, first-Thursday rule) - the international standard. Format result cells as Date or Time so they don’t display raw serial numbers.
Practice workbook
Frequently asked questions
How is ISO week 1 defined?
Why does Jan 1 sometimes show week 52 or 53?
ISOWEEKNUM vs WEEKNUM(date,21)?
Which Excel versions have it?
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