ISO week number calculator
Which ISO week a date falls in, and which dates a week covers.
Free, no signup, and it runs entirely in your browser.
2026-W37
Week 37 of 2026
Week 37 of 2026 runs Mon, 7 Sept 2026 to Sun, 13 Sept 2026
2026 has 53 ISO weeks.
About this tool
ISO 8601 weeks start on Monday, and week 1 is the week containing the first Thursday of the year. That definition has a consequence people trip over constantly: the first days of January can belong to week 52 or 53 of the previous year, and the last days of December can belong to week 1 of the next one.
- Both directions: a date gives you its week, and a year and week give you the Monday-to-Sunday range it covers.
- Most years have 52 weeks; a year has 53 when it starts on a Thursday, or on a Wednesday in a leap year.
- The year in an ISO week date is the week-numbering year, which is not always the calendar year of the date. 1 January 2027 falls in 2026-W53.
- Calculations use UTC, so a date does not shift a week because of the timezone you are in.
Questions
- How is the ISO week number defined?
- Weeks run Monday to Sunday, and week 1 is the week containing the first Thursday of the year — equivalently, the week containing 4 January.
- Can a year have 53 weeks?
- Yes. A year has 53 ISO weeks when 1 January falls on a Thursday, or on a Wednesday in a leap year. Otherwise it has 52.
- Why is 1 January sometimes in week 52?
- Because a week belongs to whichever year holds most of it. If 1 January is a Friday, that week has only three days in the new year, so it stays with the old one.