Understand weeks near the start and end of a year
ISO weeks start on Monday. Week 1 contains the first Thursday of the year. Because of this rule, dates near New Year can belong to a week year different from their calendar year.
The tool shows the week number and week year separately. Use both when naming weekly reports or comparing dates across a year boundary.
How to use this tool
- Select a calendar date.
- Read its ISO week number.
- Keep the week year with the number when recording it.
Example
1 January 2021 belongs to ISO week 53 of 2020. Reading only the calendar year would give the wrong label for that weekly report.
Common questions
Do all calendars use ISO week numbers?
No. Other systems use different week starts and first week rules. This calculator specifically uses ISO 8601.