ISOWEEKNUM function
Quick answer
ISOWEEKNUM returns the ISO 8601 week number of a date: weeks start on Monday, and week 1 is the week that contains the year's first Thursday.
- Syntax
=ISOWEEKNUM(date)- Most common use
=ISOWEEKNUM(A2)returns 1 (January 1, 2026 is a Thursday, so it starts week 1)
It is the standard in much of Europe and in many company calendars, and it can put early January dates in week 52 or 53 of the year before.
- Available in
- Excel 2013 and later
- Category
- Date functions
- Returns
- A whole number from 1 to 53.
- Google Sheets
- Works the same in Google Sheets.
Syntax
=ISOWEEKNUM(date)
| Argument | Required? | What it is |
|---|---|---|
date | Required | The date to number. |
Examples
Each result below is calculated from the sample sheet by our formula engine, the same one that grades the practice.
| Cell | Formula | Result | What it shows |
|---|---|---|---|
| B2 | =ISOWEEKNUM(A2) | 1 | January 1, 2026 is a Thursday, so it starts week 1 |
| B3 | =ISOWEEKNUM(A3) | 53 | January 1, 2027 is a Friday: it belongs to week 53 of 2026 |
| B4 | =WEEKNUM(A3) | 1 | WEEKNUM's default system calls the same day week 1 |
Try it live
This is the sample sheet. Change any number or text, or edit a formula, and the results update.
Show the sample data as a table
| Date | Result |
| 1/1/2026 | 1 |
| 1/1/2027 | 53 |
| 12/31/2026 | 1 |
Practice ISOWEEKNUM
Graded instantly, then re-checked on changed data.
ISO week numbers
B2, return the ISO week number of each date in A2. Fill down to B7.Common mistakes
| What you see | Why, and how to fix it |
|---|---|
| Week 53 in January | Correct for ISO weeks: January 1–3 can belong to the last week of the previous year. |
| Doesn't match WEEKNUM | WEEKNUM uses a different system by default. WEEKNUM(date,21) gives ISO weeks too. |
Tips
- The ISO year of a date is the year of its Thursday: =YEAR(A2-WEEKDAY(A2,2)+4).
Questions
How do I get ISO week numbers in Excel?
=ISOWEEKNUM(A2), or =WEEKNUM(A2,21) in versions before Excel 2013.
Why is January 1 in week 53?
In the ISO system a week belongs to the year that holds its Thursday. If January 1 falls on a Friday, Saturday or Sunday, it belongs to the last week of the previous year.
Updated Checked by running: every formula on this page was run by our formula engine on .