What you get
For any date the calculator shows its ISO week number and week-year, the day of the year, the quarter, how many days are left in the year, the Monday and Sunday of its week, and whether the year is a leap year. The reverse mode turns a week number into real dates, which is useful for project plans and production calendars.
Week numbering around the world
Most of Europe and many businesses use ISO weeks. North America often counts from Sunday. Check which one your calendar or school uses. For counting days use the date difference calculator, to shift a date use the add or subtract days calculator, and for business days see the working days calculator.
Frequently asked questions
What is an ISO week number?
ISO 8601 numbers weeks from 1 to 52 or 53. Weeks start on Monday, and week 1 is the week that contains the first Thursday of the year (equivalently, the week with 4 January).
Why can the week number belong to a different year?
Because weeks run Monday to Sunday, the first days of January can belong to week 52 or 53 of the previous year, and the last days of December can belong to week 1 of the next year. 31 December 2024, for example, is in week 1 of 2025.
How many weeks are in a year?
An ISO year has 52 or 53 weeks. It has 53 when 1 January is a Thursday, or when it is a leap year and 1 January is a Wednesday.
Is the US week number the same?
Not always. The US system commonly starts weeks on Sunday and calls the week containing 1 January week 1. The tool shows both so you can match the calendar you use.
How do I find the dates of a given week?
Switch the mode to 'Week number to dates', then enter the ISO year and week. You get its Monday and Sunday and every day in between.