ISO week numbers for 2025: week 1 starts on December 30, 2024 and the last week, week 52, ends on December 28, 2025. It is a regular 52-week year. The table lists every week with its Monday-to-Sunday dates; copy or download it as a list.
How to use
- The top card shows the current week number and its dates. Switch between ISO 8601 and US numbering; your choice is remembered.
- Under Date → week, pick any date to see its week number and the dates of that week.
- Under Week → dates, enter a year and a week number to list its seven days.
- Tap a week number in the calendar to select that week, or a day to look it up. Tap any row to copy it, or copy and download the whole list of weeks.
- ISO 8601 (used in Europe and most of Asia): weeks start on Monday and week 1 is the week that contains the first Thursday of the year, so it always includes January 4. A year has 52 or 53 weeks.
- Because of that rule, December 29–31 can belong to week 1 of the next year, and January 1–3 can belong to week 52 or 53 of the previous year.
- US numbering: weeks start on Sunday and week 1 is the week that contains January 1. A year has 53 weeks, or 54 in rare leap years such as 2028.
- A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday.
How ISO week numbers work
ISO 8601 weeks start on Monday, and week 1 is the week that contains the first Thursday of the year, which is the same as the week containing January 4. A year therefore has 52 or 53 weeks. A year gets 53 weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday: 2026 is one, 2032 is the next.
Because of this rule, December 29–31 can already be week 1 of the next year, and January 1–3 can still be week 52 or 53 of the previous one. In 2027, January 1–3 belong to week 53 of 2026.
US week numbers
In the US, weeks usually start on Sunday and week 1 is simply the week that contains January 1. That gives 53 weeks in most years and 54 in rare leap years such as 2028, and the numbers can differ from ISO by one. Check which system a schedule, payroll or school calendar uses before comparing week numbers.
Frequently asked questions
What week number is it today?
The number at the top of the page is today’s week, with its Monday-to-Sunday dates. It updates automatically every week.
How many weeks are in a year?
Under ISO 8601 a year has 52 or 53 weeks. 2026 has 53, while 2025, 2027 and 2028 have 52.
When does week 1 start?
ISO week 1 is the week with the first Thursday of January, so it starts between December 29 and January 4. In 2027 it starts on Monday, January 4.
Why does my calendar show a different week number?
It probably uses US numbering, where weeks start on Sunday and week 1 contains January 1. Switch the numbering on this page to compare.
How do I find the dates of a given week?
Open Week → dates, enter the year and week number, and the seven days of that week are listed. Tap a row to copy it.