Schools, employers and shipping companies in Europe plan by week number. Here is how the ISO system defines weeks, why week 1 can begin in the previous December and why some years have 53 weeks.
What the ISO standard says
The international standard ISO 8601 defines a week as running from Monday to Sunday and numbers the weeks from 1 to 52 or 53. The rule for week 1 is that it is the week that contains the first Thursday of the year. Another way to say the same thing is that week 1 is the week with January 4, or the first week that has at least four days in the new year.
Norway, Sweden, Germany and most of Europe use this system in calendars, school plans and workplaces.
Why Thursday?
The point of the rule is that each week belongs to exactly one year. A week that runs from Monday to Sunday can be split between two calendar years, and the rule decides which year gets it. Because Thursday is in the middle of the week, a week with its Thursday in the new year has at least four of its seven days in that year, so it is counted as belonging there.
The result is that 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.
Examples near New Year
- December 30, 2024 was a Monday and belongs to week 1 of 2025, because the Thursday of that week is January 2, 2025.
- January 1, 2027 is a Friday and belongs to week 53 of 2026, because the Thursday of that week is December 31, 2026.
- December 29, 2024 was a Sunday and is the last day of week 52 of 2024.
When you write a date with a week number, it is therefore safest to add the ISO year as well, for example 2025-W01, so there is no confusion about which year the week belongs to.
Years with 53 weeks
Most years have 52 weeks, but a year has 53 weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. The recent and upcoming years with 53 weeks are 2020, 2026, 2032 and 2037.
The reason is that 365 days is 52 weeks and one day, and 366 days is 52 weeks and two days. The extra days build up until a whole extra week is needed, which happens on average every five or six years.
How to find the week number of a date
To work it out by hand, find the Thursday of the week your date is in, and then count how many Thursdays have occurred in that year up to and including that one. The count is the week number. If the Thursday falls in the previous or next year, the week belongs to that year.
The Thursday method is easy to remember and always gives the right answer.
Common questions
Does week 1 always start in January? No. In some years it starts on the last Monday of December, as in 2024, when week 1 of 2025 began on December 30. In other years it starts as late as January 4, when the first Monday of the year is the fourth.
Is the week number the same everywhere in Europe? Almost. Countries that follow ISO 8601 agree on every date. Differences occur only with companies or countries that use their own rules, so check if the week number matters for a contract or delivery.
How many weeks are left in the year? Count forward to December 28, which is always in the last week of its ISO year, and read its week number to see whether the year ends at 52 or 53.
Other week numbering systems
The United States, Canada and some other countries commonly use a different system. Weeks begin on Sunday, and week 1 is the week that contains January 1. In that system the week numbers can differ by one from the ISO numbers, and the first week of the year can be a short one with only a day or two.
In the Middle East, weeks traditionally begin on Saturday. When you work with international colleagues it is a good idea to say which system you mean, or to give dates instead of week numbers.
Week numbers in practice
Week numbers are used in school calendars, in production and shipping schedules, and in project plans. A statement such as delivery in week 42 is short and unambiguous within one country that uses ISO numbering. For a deadline that crosses borders, use the exact date.
On this site, the week-number tool shows the week number for today and for any date you want to check.