ISO Week Calculator
Find the ISO week number, week-year and weekday for a calendar date, including year-boundary weeks.
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How it works
Paste your content or load the example. Choose the available options, run the tool, then copy or download the result.
A practical example
Dates · ISO 8601
2026-01-01
Result
2026-W01-4
Frequently asked questions
Is my input sent to a server?
No. Your content is processed in this browser. It is never sent to the server or saved.
What should I know about ISO Week Calculator?
Use YYYY-MM-DD. ISO weeks start on Monday; the first week contains the first Thursday. The week-year can differ from the calendar year.