Tool
ISO Week Number Calculator
Find the ISO 8601 week number for any date. See which week of the year a date falls in, along with the week's Monday-to-Sunday range.
Week Number Calculator
ISO Week Number
W24
ISO Year
2026
Day of Week
Sunday
Sunday, June 14, 2026 falls in week 24 of 2026.
This week runs from Monday, June 8, 2026 to Sunday, June 14, 2026.
Frequently asked questions
- What is an ISO week number?
- ISO 8601 defines week numbers where Week 1 is the week containing the first Thursday of January. Weeks always start on Monday and end on Sunday. This system is standard in Europe and commonly used in project management.
- Why can Week 1 start in December?
- If January 1 falls on a Friday, Saturday, or Sunday, the first Thursday of January is in the following week — so the days before Thursday belong to the last week of the previous year (Week 52 or 53).
- Can a year have 53 weeks?
- Yes. A year has 53 ISO weeks when January 1 falls on a Thursday (or Wednesday in a leap year). Most years have 52 weeks.
- How is the ISO year different from the calendar year?
- The ISO year (also called the week date year) is the year that the majority of the week's days belong to. For example, December 31, 2025 might be in ISO week year 2026 if that week's Thursday falls in January 2026.