Week Number Calculator
Find the ISO 8601 week number for a date — the international standard used across Europe and by most calendar and project management tools.
How to use the week number calculator
- Choose the date you want to find the week number for.
- The calculator shows the week number according to the ISO 8601 standard.
How the week number is calculated
ISO 8601 defines week 1 as the week containing the year's first Thursday. This means the Monday of week 1 always falls in the same calendar year, or at the end of December the previous year:
The week always starts on Monday
This is why the first days of January can sometimes belong to the last week of the previous year, and the last days of December can belong to week 1 of the following year.
ISO weeks — why 1 January can fall in week 52
Most of Europe follows ISO 8601, where the week starts on Monday and week 1 is defined as the week containing the year's first Thursday. The consequence is that the first days of January can belong to the last week of the preceding year, and the last days of December can belong to week 1 of the coming year. That is not a flaw in the calendar but a direct result of the definition.
For the same reason, some years have 53 weeks rather than 52. That happens when the year starts on a Thursday, or when a leap year starts on a Wednesday. Systems that assume a year always has 52 weeks therefore run into trouble at regular intervals — and calendars in the United States, which commonly start the week on Sunday and use a different rule for week 1, produce yet another set of numbers.
Frequently asked questions
Why do some years have 53 weeks?
Most years have 52 weeks, but years where January 1st falls on a Thursday (or a leap year where it falls on a Wednesday) get 53 weeks according to ISO 8601.
Can week 1 start in December of the previous year?
Yes. If January 1st falls on a Friday, Saturday or Sunday, the Monday of week 1 falls at the end of December the previous year.