Week number
ISO week number is not the same as a US Sunday-first calendar. Weeks start on Monday. Week 1 is the week that contains 4 January. Early January can still be week 52 or 53 of the previous ISO year. This page does not replace the weekday or day-of-the-year tools.
What ISO week number is this date
Day-of-year counts live on the day of the year tool. Weekday names live on the weekday calculator.
Week number FAQ
Which week numbering is this?
ISO-8601. Weeks start on Monday. Week 1 is the week that contains 4 January. The US month grids on this site stay Sunday-first.
Can the week year differ from the calendar year?
Yes. Early January can still be week 52 or 53 of the previous ISO year, and late December can be week 1 of the next.
When is there a week 53?
An ISO year has 53 weeks when 1 January is a Thursday, or a Wednesday in a leap year. Use Same week to list those years.
Can I compare several dates?
Yes. Add dates, or look up a week number by year. Save, CSV, and print need scripting; today’s ISO week stays in the HTML.