Week Number Calculator
Enter a date to find its ISO 8601 week number.
How It Works
ISO 8601 weeks start on Monday, and week 1 of a year is defined as the week containing that year's first Thursday (equivalently, the week containing January 4th). This means the first few days of January can belong to the last week of the previous year, and the last few days of December can belong to week 1 of the next year.
Example
January 4th always falls in week 1 of its year, by definition.
Frequently Asked Questions
- Why might January 1st show as a week from the previous year?
- If January 1st falls on a Friday, Saturday or Sunday, it belongs to the last ISO week of the previous year rather than week 1 of the new year.
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