ISO Week Calculator
Find ISO week number, week-based year, weekday, and Monday–Sunday dates for a calendar date.
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How to use ISO Week Calculator
ISO weeks start on Monday. Week 1 is the week containing January 4, equivalently the year’s first Thursday. The ISO week-year can differ from the calendar year.
- Enter calendar date.
- Check that the input follows the stated limits and uses consistent units.
- Run the calculator, review the result, then copy or download it.
Good to know
- Uses the proleptic Gregorian calendar for years 0001–9999. This is ISO week numbering, not a local Sunday-first or retail fiscal calendar.
- Calendar dates are processed locally in UTC with Gregorian leap-year rules. No user input or calculated dates are saved by the tool.
Common questions
What calculation does ISO Week Calculator perform?
ISO weeks start on Monday. Week 1 is the week containing January 4, equivalently the year’s first Thursday. The ISO week-year can differ from the calendar year.
Why can January 1 belong to the previous year’s week?
An ISO week belongs to the year containing its Thursday, so dates near New Year may have a different week-based year.
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