📅 ISO Week Number Calculator — Week of the Year
Pick a date to see its ISO week number. Weeks start on Monday, and week 1 is the week that contains the first Thursday of the calendar year.
How it works
- Choose a calendar date. The default is 5 October 2026.
- Press Get week number. The result uses the ISO rule that weeks start on Monday.
- Week 1 is the week with the year's first Thursday, so early January can belong to the previous week-year.
Examples
- 1 January 2021 was a Friday in ISO week 53 of 2020.
- 4 January 2021 was a Monday and the start of ISO week 1 of 2021.
- 5 October 2026 falls in an ISO week that starts on the previous Monday.
Frequently asked questions
Why can January belong to the previous year?
ISO week-years follow Thursday. If 1 January falls on Friday, Saturday or Sunday, those days are in week 52 or 53 of the previous week-year.
Which day starts the week?
Monday. Sunday is the last day of an ISO week, not the first.
Does this use my timezone?
The date you pick is treated as a calendar date, not a timestamp, so timezone offsets do not shift the week.
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