Week Number Calculator (ISO 8601)
Enter a date to find its ISO week number and the day of the year. You can also find the date from an ISO week, or from a year and a day of the year.
This calculator uses the ISO 8601 week system
There are several ways to number weeks. This calculator uses the ISO 8601 method (the week date). It is not the numbering of a US calendar that starts the week on Sunday, and it is not "the week containing January 1 is week 1".
- The day of the year counts from January 1 as day 1 (October 29, 2026 is day 302).
- A week starts on Monday and ends on Sunday.
- Week 1 is the week that contains the first Thursday of the year. Put another way, week 1 is the week that contains January 4.
- A week belongs to the year that contains its Thursday. That is the ISO week-year.
- It is written as week-year, W, week number, weekday number (1 for Monday to 7 for Sunday), for example
2026-W44-4(Thursday of week 44 of 2026).
Days where the calendar year and the ISO week-year differ
Around New Year, the calendar year and the ISO week-year can differ, because the week-year is decided by the year of the week's Thursday.
| Date | ISO week | Explanation |
|---|---|---|
| 2005-01-01 (Saturday) | 2004-W53-6 | It falls in the last week of the previous year |
| 2010-01-03 (Sunday) | 2009-W53-7 | It falls in the last week of the previous year |
| 2008-12-29 (Monday) | 2009-W01-1 | It falls in week 1 of the next year |
| 2024-12-30 (Monday) | 2025-W01-1 | It falls in week 1 of the next year |
These values match the examples on the Wikipedia page "ISO week date" (checked October 31, 2026).
Years with 53 weeks
An ISO week-year has either 52 or 53 weeks. A year that starts on a Thursday, and a leap year that starts on a Wednesday, have 53 weeks (2026 has 53 weeks).
Three calculations
- From a date: the day of the year, the number of days in the year and the days left, the ISO week number, the weekday, the Monday-to-Sunday dates of that week and the number of weeks in that week-year.
- Date from an ISO week: the date from a week-year, a week number and a weekday. A week that does not exist (week 53 in a 52-week year) gives an error.
- Date from a day of the year: the date from a year and a day of the year.
Dates from January 1, 1900 to December 31, 2100 can be used (1900-01-01 is 1900-W01-1, and 2100-12-31 is 2100-W52-5).
What this calculator does not do
- Week numbers other than ISO (weeks that start on Sunday, or the week containing January 1 being week 1)
- The week of a month, quarters and fiscal years
- Times and time zones
The rules above were checked against two explanatory sources: the Wikipedia page "ISO week date" and the documentation of Python's date.isocalendar(). The text of the ISO 8601 standard itself has not been checked.
Sources and checks
- Wikipedia, ISO week date (checked October 31, 2026): weeks start on Monday, week 1 contains the first Thursday (and always January 4), a week belongs to the year of its Thursday, the 53-week years, and the New Year examples in the table above.
- Python documentation, datetime: date.isocalendar() (checked October 31, 2026): "a week starts on a Monday and ends on a Sunday", "the first week of an ISO year is the first (Gregorian) calendar week of a year containing a Thursday", and an ISO year has 52 or 53 full weeks.
- This calculator was also compared with Python's
date.isocalendar()for every date from 1900 to 2100 (73,414 dates): the ISO year, week number and weekday all matched, the reverse conversion (from the ISO week back to the date) returned the same date, and the number of weeks in each year (52 or 53) matched for all 201 years.
Frequently asked questions
Can the ISO week number be 53?
Yes. It is the last week of a year that has 53 weeks. Also, the first days of January can fall in week 52 or 53 of the previous year.
Why is January 1 sometimes in the previous year's week?
Week 1 is the week with the first Thursday. When January 1 is a Friday, Saturday or Sunday, the Thursday of its week is in the previous year, so that week belongs to the previous year.
What about the difference between dates, or adding days to a date?
Use the Days Between Dates Calculator or the Date Calculator. For Unix time, use the Unix Timestamp Converter.