Week Number Calculator
Find the ISO-8601 week number of any date, and the dates that week runs from and to.
About the Week Number Calculator
In the ISO system weeks start on Monday, and week 1 is the week containing the year's first Thursday (equivalently, 4 January). So 1–3 January can belong to the last week of the previous year, and a year has 52 or 53 weeks. The US often numbers weeks differently, starting on Sunday with the week containing 1 January.
The formula
Week 1 = the week with the year's first Thursday
Worked example
Example: ISO week number = Week 11
- Input: Date: 15 March 2026.
- Formula: Week 1 = the week with the year's first Thursday
- Result: ISO week number = Week 11
- ISO week date: 2026-W11-7
Questions people ask
Why does 31 December show week 1?
If that week contains the next year's first Thursday, it belongs to the next ISO year.
Which years have 53 weeks?
Years starting on a Thursday, and leap years starting on a Wednesday.