The WEEKNUM function
Returns the week number of a date. By default weeks start on Sunday and week 1 is the week with Jan 1 in it; return type 21 gives ISO week numbers, as used in Europe.
Syntax
WEEKNUM(serial_number, [return_type])- serial_number
- The date to check.
- return_type (optional)
- 1 or left out for weeks starting on Sunday, 2 for weeks starting on Monday, 21 for ISO weeks (Monday start, week 1 holds the first Thursday).
Examples
B1=WEEKNUM(DATE(2024,3,15))
| A | B | |
|---|---|---|
| 1 | 11 |
B1=WEEKNUM(DATE(2021,1,1))
| A | B | |
|---|---|---|
| 1 | 1 |
B1
=WEEKNUM(1DATE(2021,1,1),221)| A | B | |
|---|---|---|
| 1 | 53 |
In plain words: The week number of 1Jan 1, 2021, 2counted the ISO way.
Questions
How do I get ISO week numbers in Excel?
Use =WEEKNUM(A2,21) or =ISOWEEKNUM(A2). ISO weeks start on Monday, and week 1 is the week that holds the first Thursday of the year.
Why is Jan 1 in week 53?
With ISO weeks, the first days of January can belong to the last week of the year before. Jan 1, 2021 was a Friday, so it is in week 53 of 2020.