Week Number Calculator - ISO 8601 Week of the Year for Any Date
Week Number Calculator
ISO and US week numbers, and the dates of any week
ISO 8601 Weeks
The week numbering used in Europe, in most business software and by Outlook in many countries.
US Weeks Too
See the Sunday-start numbering where week 1 always contains 1 January.
Both Directions
Turn "week 23" back into its Monday-to-Sunday dates.
Private
Calculated in your browser.
How ISO Week Numbers Work
Under ISO 8601, weeks run Monday to Sunday and week 1 is the week that contains the year's first Thursday (equivalently, the week with 4 January). That is why a few days at the edges of a year belong to the neighbouring year's weeks: 1 January 2027 is a Friday, so it falls in week 53 of 2026, written 2026-W53-5. Most years have 52 ISO weeks; those starting on a Thursday, and leap years starting on a Wednesday, have 53.
In the United States, calendars usually start weeks on Sunday and call the week containing 1 January week 1, so the two numbers can differ by one for the same date. When you share week numbers across countries, say which system you mean, or use the unambiguous ISO notation such as 2026-W40.
Key Takeaways
- Monday start: ISO weeks always begin on Monday.
- 52 or 53 weeks: Some years have a week 53; the calculator tells you which.
- Year boundaries: Early January can belong to the previous ISO year.