Week Number Calculator
Find the ISO week number, day of year and quarter for any date.
Find the ISO-8601 week number for any date, along with the day of the year, the calendar quarter and how many days remain in the year. ISO weeks start on Monday, and week 1 is the week containing the year's first Thursday — which is why early-January dates sometimes belong to the previous year's last week.
Week numbers run business, manufacturing and project planning worldwide, especially in Europe where dates are often quoted as "week 32". This tool removes the guesswork.
Frequently asked questions
How is the ISO week number calculated?
Weeks start on Monday and week 1 is the week containing the year's first Thursday (equivalently, the week with 4 January in it).
Why can week 1 start in December?
Because week 1 hinges on the first Thursday, a few late-December days can belong to the next year's week 1, and early-January days to the old year's last week.
What is a 53-week year?
Most years have 52 weeks, but years that start on a Thursday (or Wednesday in a leap year) have 53. It catches a lot of planning software out.
Where are week numbers used?
European business, factory schedules, academic calendars, agile sprints and shipping timelines commonly quote dates as week numbers.