ISO Week Number Calculator
Determine the ISO 8601 week number for a given date. Enter year, month, and day.
Uses the Gregorian calendar and ISO 8601 definition of week numbers.
Examples
- 2023-09-01 ⇒ 35
- 2025-01-01 ⇒ 1
FAQ
What is ISO week numbering?
It's a standard where weeks start on Monday and week 1 is the week with the year's first Thursday.
Are week numbers always 1–52?
Some years have 53 ISO weeks.
Do I need to account for time zones?
The calculation uses UTC to avoid time zone issues.
Can it handle leap years?
Yes, the Date object manages leap years automatically.
Additional Information
- ISO weeks start on Monday, and week 1 contains the year's first Thursday.
- Week numbers can span across calendar years; early January dates may belong to the last week of the previous year.
- Some years include a 53rd week depending on how the year aligns with the week cycle.
- Using ISO week numbers helps standardize schedules across countries and industries.