Week Number Calculator
Find the ISO 8601 week number of any date, or the start and end dates of a given week.
Date → week number
Week number → dates
Uses ISO 8601: weeks start on Monday and week 1 contains the first Thursday of the year.
Which week of the year are we in?
Production planning, logistics, retail reporting, sprint planning and school calendars often refer to dates by week number. The Week Number Calculator shows the ISO 8601 week number of any date instantly.
ISO week rules
Under ISO 8601, weeks start on Monday and week 1 is the week containing the year's first Thursday. That's why some years have 53 weeks and early January days can belong to the previous year's last week.
Extra information
- Start and end dates of a given week number
- Day of the year
- Quarter (Q1–Q4)
- Total weeks in the year
How to use Week Number Calculator
- Pick a date or enter a year and week number.
- See the ISO week number or the week's date range.
- Review the day of the year and quarter.
- Use the result in your planning.
Why use this tool?
Avoid reporting and planning errors with internationally standard week numbers.
FAQ
Why does 1 January sometimes belong to week 52 or 53?
If 1 January falls on a Friday, Saturday or Sunday, those days belong to the previous year's last ISO week.
How do I get the ISO week number in Excel?
The ISOWEEKNUM function returns the ISO 8601 week number.
Are US week numbers different?
Yes. In the US system weeks start on Sunday and 1 January is always in week 1, so results can differ.
Related tools
Date Calculator (Add or Subtract Days)
Add or subtract days, weeks, months and years to a date and see the result and weekday.
Open toolDays Between Dates Calculator
Count the days between two dates and see them as weeks, months, years, hours and weekdays.
Open toolBusiness Days Calculator
Count working days between two dates or add working days to a date, skipping weekends and your holidays.
Open toolTime Zone Converter
Convert a date and time between two time zones; UTC offsets and daylight saving time are handled automatically.
Open tool