Week number calculator
Calculator
The calculator needs JavaScript. The tables below give the same numbers for common cases.
How it is worked out
- ISO 8601: weeks run Monday to Sunday and week 1 contains the first Thursday of the year.
- A date in late December or early January can belong to a week of the neighbouring year.
| Inputs | Answer |
|---|---|
| Date: 2026-11-13 | Friday, November 13, 2026 is in ISO week 46 of 2026. |
| Date: 2027-01-01 | Friday, January 1, 2027 is in ISO week 53 of 2026. |
| Date: 2026-12-31 | Thursday, December 31, 2026 is in ISO week 53 of 2026. |
How to use it
The ISO 8601 week number counts weeks that run from Monday to Sunday; week 1 is the week with the first Thursday of the year. Because of this, 1 January can fall in week 52 or 53 of the previous year, and late December can fall in week 1 of the next.
The calculator also shows the dates of the Monday and Sunday of that week.
Frequently asked questions
What week number is it?
Enter the date to see its ISO week.
Why is 1 January sometimes in week 52?
ISO week 1 contains the first Thursday, so early January days can belong to the last week of the previous year.
Do all countries use ISO weeks?
No; the US often numbers weeks differently, starting on Sunday.
Method: standard arithmetic shown step by step; no outside data is used. Checked 2026-11-12.
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General information only. Results are plain arithmetic on the numbers you enter; check anything important (money, health, school work) against your own records or the official source.