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Week number calculator

The ISO week number for any date, the dates of any week, and leap years. Weeks start on Monday.

I want to
Week number 
Weeks in
Day of the year
Is a leap year?
Next leap years
All weeks in
WeekMondaySunday

Week numbers follow ISO 8601 and use your device's date. Some US calendars number weeks differently. Terms of use.

Illustration for the week number calculator

How ISO week numbers work

ISO 8601 is the international standard for dates. Its weeks start on Monday, and week 1 of a year is the week that contains the first Thursday in January. A year has 52 or 53 weeks, and a week near New Year can belong to the year before or after its Monday.

Example. Week 1 of 2026 runs from Mon 29 December 2025 to Sun 4 January 2026. 1 January 2027 falls in week 53 of 2026.

Leap years

Leap years keep the calendar in line with the Earth's orbit, which takes about 365.2425 days. Every fourth year has a 29 February, except century years that do not divide by 400. The next leap years are 2028, 2032, 2036.

Questions

What week number is it?
The calculator shows the ISO week number for today when the page opens. ISO weeks run Monday to Sunday and are the week numbers used in most of Europe, including Ireland.
How is week 1 decided?
Under ISO 8601, week 1 is the week with the first Thursday of the year in it. Another way to say the same thing: week 1 is the week that contains 4 January. So 1 to 3 January can fall in the last week of the year before.
Why do some years have 53 weeks?
A year has 53 ISO weeks when it starts on a Thursday, or on a Wednesday in a leap year. 2026 has 53 weeks and 2027 has 52.
What is a leap year?
A leap year has 366 days, with 29 February added. A year is a leap year if it divides by 4, except years ending in 00, which must also divide by 400. So 2000 was a leap year and 1900 was not.
When is the next leap year?
The next leap years after 2026 are 2028, 2032, 2036.

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