Find the ISO week number of any date, the US week number and day of the year, or look up the Monday-to-Sunday dates of any week in any year.
About this tool
Find the ISO week number of any date, and the dates of any week of the year.
How to use Week Number Calculator
- Choose Week of a date or Dates of a week.
- Enter a date, or a year and week number.
- Read the week number or the dates of that week.
Worked example
30 September 2026 is in ISO week 40. Week 1 of 2027 starts on Monday 4 January.
How are ISO weeks counted?
Weeks start on Monday, and week 1 is the week containing the year’s first Thursday.
Can a year have 53 weeks?
Yes. Years that start on a Thursday, or leap years that start on a Wednesday, have 53 ISO weeks.
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