ISO Week Number Calculator
Find the ISO 8601 week number for any date. See week starts and ends. Free online week number tool.
ISO 8601: weeks start Monday, week 1 contains the first Thursday of the year.
Frequently asked questions
- What is an ISO week number?
- A standard where week 1 is the week containing the first Thursday of January, and every week starts on Monday.
- Why can January dates belong to the last week of the previous year?
- Because ISO weeks are counted by Thursdays. January 1 sometimes falls in week 52 or 53 of the year before.
- Do all countries use this system?
- Most of Europe and much of Asia use ISO weeks. The US more often refers to calendar months and quarters.
- How many weeks can a year have?
- 52 or 53. A year has 53 ISO weeks when it starts on a Thursday, or a leap year starting on Wednesday.
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