WEEK NUMBER GUIDE
An ISO week number is a standardized way to number the weeks of the year, defined by the ISO 8601 international standard. Week 1 is the week containing the first Thursday of the year — and every week runs Monday to Sunday.
ISO week numbering assigns a number from 1 to 52 (or 53) to each week of the year. It is defined by the ISO 8601 standard, used in business, logistics, and government across Europe and internationally.
Key rules:
Week 1 is the week that contains January 4. This is equivalent to saying: the week containing the first Thursday of the year.
This means January 1 is sometimes in week 52 or 53 of the previous year. For example:
Conversely, late December days can fall in week 1 of the next year. December 31, 2018 was a Monday — it was in week 1 of 2019.
A regular year has 52 weeks and 1 extra day. A leap year has 52 weeks and 2 extra days. When January 1 falls on a Thursday, the year has 53 ISO weeks. Leap years that start on a Wednesday also get 53 weeks.
Recent years with 53 ISO weeks: 2015, 2020, 2026, 2032.
| ISO 8601 (international) | US / Canada | |
|---|---|---|
| First day of week | Monday | Sunday |
| Week 1 definition | Contains January 4 (first Thursday) | Contains January 1 |
| Used in | Europe, business, logistics | USA, Canada, Japan |
| Years with 53 weeks | Years starting on Thu (or Wed in leap) | Years starting on Sun (or Sat in leap) |
Use the week number calculator on this site — it shows the current ISO week number updated in real time.
No. ISO week 1 starts on Monday of the week containing January 4. January 1 can fall in week 52 or 53 of the previous year if it is a Friday, Saturday, or Sunday.
2025 has 52 ISO weeks. Week 1 of 2025 started on Monday, December 30, 2024. Week 52 ends on Sunday, December 28, 2025.
2026 has 53 ISO weeks, because January 1, 2026 falls on a Thursday. Week 53 of 2026 contains December 28–January 3, 2027.
"Week number" and "calendar week" (KW in German, CW in English) both refer to ISO week numbers — they are the same thing. The term "calendar week" is common in German-speaking countries and business contexts.