Date to ISO week number converter
Pick a date to see which ISO week it falls in, the numbering used across Europe, business calendars and many planning tools.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter date.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Take the Thursday of the same week: the ISO year is that Thursday's year, and week = floor((Thursday's day of year - 1) / 7) + 1.
Example
Inputs: Date = 2026-10-01. Result: ISO week: 2026-W40; Week number: 40; Week, Monday to Sunday: 2026-09-28 to 2026-10-04.
Limits and caveats
- It gives the ISO 8601 week only; Excel's default WEEKNUM can differ, so compare with ISOWEEKNUM.
- Day of year is counted in the calendar year of the date, not the ISO year.
Questions about this tool
Why is 1 January sometimes in week 52 or 53?
A week belongs to the year of its Thursday. If 1 January is a Friday, Saturday or Sunday, it is in the last week of the previous year.
What does the last digit in 2026-W40-4 mean?
It is the day of the week, 1 for Monday to 7 for Sunday, so 4 is Thursday.
How do I get the same number in Excel?
Use =ISOWEEKNUM(A1) in Excel 2013 or later, or in Google Sheets.