Week Number Calculator
Find the ISO 8601 week number for any date.
Last updated
How to find the week number of a date
- Pick a date, or keep today’s.
- Read the ISO week number and the ISO week-year it belongs to.
About this tool
ISO 8601 weeks run Monday to Sunday, and week 1 of a year is defined as the week containing that year's first Thursday — equivalently, the week containing January 4th. Because of this, the first few days of January can fall in the final week (52 or 53) of the previous ISO year, and the last few days of December can fall in week 1 of the next one — which is why this tool shows an "ISO week-year" separately from the calendar year. Most years have 52 weeks; years where January 1st is a Thursday (or a leap year where it's a Wednesday) have 53.
Nothing entered here is sent to a server — the entire calculation runs as JavaScript in your browser tab.
Questions about week numbers
What week number is it?
Open the page and today’s ISO week number is shown. ISO weeks run Monday to Sunday and are what most of Europe and business calendars use.
How are ISO week numbers decided?
Week 1 is the week with the year’s first Thursday in it, which is always the week containing 4 January. Weeks start on Monday.
Why are some early January days in week 52 or 53?
If 1 January falls on a Friday, Saturday or Sunday, those days belong to the last week of the previous year. That’s why the ISO week-year is shown separately.
Which years have 53 weeks?
Years that start on a Thursday, and leap years that start on a Wednesday. 2026 is one: it began on a Thursday.
Are US week numbers the same?
Not always. A common US system starts weeks on Sunday and counts the week containing 1 January as week 1, so it can differ by one from ISO.