The Excel WEEKNUM function returns the week number of the year — handy for weekly reporting and grouping — using the simple “week containing January 1 is week 1” system.
Syntax
| Argument | Description | |
|---|---|---|
serial_number | Required | The date. |
return_type | Optional | Week start: 1 = week with Jan 1 is week 1, Sunday start (default); 2 = Monday start; 21 = ISO week. |
How to use it
WEEKNUM uses the US convention: week 1 is whatever week contains January 1. For the international standard (weeks start Monday, week 1 contains the first Thursday), use ISOWEEKNUM instead — the two disagree near New Year.
Try it: interactive demo
Adjust the input and watch the formula and result update.
Tips & gotchas
Week number of the year (US convention) for weekly reporting and grouping. Format result cells as Date or Time so they don’t display raw serial numbers.
Practice workbook
Frequently asked questions
WEEKNUM vs ISOWEEKNUM?
How do I make weeks start Monday?
Why does early January show week 52 or 53?
Can I group a PivotTable by week?
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