ISO Week Number Calculator
Enter a date to get its ISO 8601 week number, ISO year, and weekday. ISO week 1 is the week that contains the first Thursday of the year.
How to find the ISO week number
- Enter the year, month, and day.
- Read the ISO week number and the ISO year.
- Note the ISO weekday, where Monday is 1 and Sunday is 7.
Examples
New Year that belongs to the previous ISO year
2027-01-01
ISO year 2026, week 53, weekday 5 (Friday).
Frequently asked questions
What is an ISO week number?
ISO 8601 numbers the weeks of the year starting from week 1, the week that contains the first Thursday of the year. Weeks run Monday to Sunday.
Why can the ISO year differ from the calendar year?
A date near the start or end of the year can fall in a week that mostly belongs to the neighbouring year. For example, 1 January 2027 is in ISO week 53 of 2026.
How many ISO weeks are in a year?
Most years have 52 ISO weeks. Some have 53, namely years that start on a Thursday or leap years that start on a Wednesday.
Which day is the first day of the ISO week?
Monday. ISO 8601 numbers weekdays from Monday as 1 through Sunday as 7, unlike the common Sunday-first convention.
Can I convert a week number back to a date?
Yes. Give an ISO year, week, and weekday and the tool returns the exact calendar date, which is useful for planning around week-based schedules.
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