ISO Week Number Calculator
Enter a date to see its ISO 8601 week number (W01 to W53), ISO year, weekday, day of the year, days left in the year and quarter. Year-boundary cases are handled correctly, such as December 30, 2024 falling in week 1 of 2025. You can also work backwards from a year and week number to the Monday-to-Sunday dates. For a printable calendar, try the Calendar Generator; for the gap between two dates or the date N days later, use the Date Calculator. Everything runs in your browser, and nothing you type is sent to a server.
🔒 Runs in your browser · Your data is never sent to a server
Look up a date
- ISO week number
- ISO year
- Weekday
- Day of the year
- Days left in the year
- Quarter
- This week
- Weeks in this ISO year
Find the dates of a week
Dates
How to use
- Enter a year, month and day under "Look up a date", or press the "Today" button.
- The ISO week number, ISO year, weekday, day of the year, days left and quarter appear.
- To go the other way, enter a year and week number under "Find the dates of a week".
Notes
- ISO 8601 weeks start on Monday, and the ISO year is the year that contains the week’s Thursday. Because of this, early January can belong to week 52 or 53 of the previous year, and late December to week 1 of the next.
- US-style week numbers (weeks start on Sunday and week 1 contains January 1) can differ from ISO numbers. This tool supports ISO 8601 only.
- Calculations use the calendar date itself, so your device’s time zone does not affect them (only the "Today" button reads your device’s date).
- Dates from year 0001 to 9999 are accepted, using the Gregorian calendar throughout.
Glossary
ISO week number (ISO 8601)
The international standard for numbering weeks. Weeks start on Monday, and week 1 is the week containing the first Thursday of the year (equivalently, the week containing January 4). A year has 52 or 53 weeks.
ISO year
The week-based year that goes with a week number. It is the calendar year of the week’s Thursday, so a few days at the start or end of a year can belong to the neighboring ISO year.
FAQ
Why is January 1, 2021 in week 53 of 2020?
ISO 8601 weeks start on Monday, and a week belongs to the year that contains its Thursday. January 1, 2021 is a Friday and its Thursday is December 31, 2020, so the date falls in week 53 of ISO year 2020.
Why do some years have 52 weeks and others 53?
A year is one or two days longer than 52 full weeks, and the extra days add up, producing a 53-week year about every five to six years. Years that start on a Thursday (or leap years that start on a Wednesday) have 53 weeks.
Why does my calendar app show a different week number?
Many apps, especially in the US, use weeks that start on Sunday with week 1 containing January 1. That differs from ISO numbering, which this tool follows exclusively, so numbers can be off by one.
Is "day of the year" based on the calendar year or the ISO year?
The calendar year. January 1 is day 1 and a leap year ends on day 366. Days left does not include the date itself, so December 31 shows 0.
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