Ask two colleagues what week of the year it is and you may get two answers, because "week 31" depends entirely on how you started counting. The ISO week number is the one definition that removes the argument: a formally specified scheme in which every week runs Monday to Sunday and every week belongs to exactly one numbered year. European manufacturing, logistics and payroll systems have run on it for decades, and its quirks catch out anyone meeting it for the first time.
This page sets out the rule that defines week 1, explains why some years contain 53 weeks rather than 52, and shows why the last days of December can legitimately sit in week 1 of the following year. The current week, along with the day of year and the full week date, is displayed on the local date and time page.
What Is an ISO Week Number?
An ISO week number identifies one of the 52 or 53 weeks of a year under the rules of ISO 8601. Weeks always begin on Monday and end on Sunday, they are numbered from 01, and week 01 is defined as the week containing the first Thursday of January.
Two equivalent statements of the same rule are often easier to apply. Week 01 is the week that contains 4 January, and it is also the first week with the majority of its days, four or more, falling in the new year. All three formulations pick out exactly the same seven days, so use whichever is easiest to check by hand. Note that the numbering has its own year, called the week-numbering year, which can differ from the calendar year for a few days at either end.
Why the First Thursday?
Thursday is the midpoint of a Monday-to-Sunday week, so the week containing the first Thursday of January is necessarily the first week with more days in the new year than the old one. Choosing the midpoint makes the rule symmetrical and free of edge cases.
The consequence is that a week is never split between two numbering years. Every week belongs wholly to one, which is what makes the scheme useful for reporting: totals for week 14 mean the same seven days to everyone, and no figure ever has to be apportioned across a year boundary. This is a real advantage over the informal alternatives, and it is worth understanding before adopting one of them by accident.
Why Do Some Years Have 53 Weeks?
Because 52 weeks account for only 364 days, a year of 365 or 366 days leaves one or two days over, and those accumulate until an extra week is needed. A year has 53 weeks when 1 January falls on a Thursday, or when it is a leap year beginning on a Wednesday.
An equivalent test is that a year is long whenever 31 December falls on a Thursday. Long years occur 71 times in every 400-year cycle, which is a little under one year in five. The year 2026 is one of them, beginning on a Thursday and therefore running to week 53; 2020 was another, being a leap year that began on a Wednesday. Any system with a hard-coded assumption of 52 weeks will produce a wrong result roughly every fifth year, and the failure typically appears in a year-end report rather than anywhere convenient.
Why Can Late December Fall in Week 1?
Because the week must belong wholly to one numbering year, and the first-Thursday rule sometimes assigns a week starting in December to the following year. Up to three days at the end of December can carry a week 01 number belonging to the next year.
A worked example makes it concrete. Monday 30 December 2019 began a week whose Thursday was 2 January 2020, so that entire week is 2020-W01, and both 30 and 31 December 2019 carry a 2020 week number. The reverse happens too: 1, 2 and 3 January 2021 all fell in 2020-W53, because that week's Thursday was 31 December 2020. Similarly, 1 to 3 January 2027 will belong to week 53 of 2026. This is not a bug in anyone's calendar, and it is the direct price of never splitting a week.
How the ISO Week Number Is Written
ISO 8601 defines a written form for these values. It runs year, then the letter W, then the two-digit week, optionally followed by a hyphen and a single digit for the weekday:
- 2026-W31: week 31 of the week-numbering year 2026, as a whole.
- 2026-W31-3: the Wednesday of that week, since day 1 is Monday and day 7 is Sunday.
- 2019-12-30 equals 2020-W01-1: the same day expressed both ways, showing the year boundary in action.
- Numbering from Monday: unlike some programming languages, which number Sunday as zero, the standard puts Monday first and Sunday last.
Written this way an ISO week number is machine-readable and sorts correctly, which is why manufacturing and logistics systems exchange schedules in it rather than in prose such as "week commencing 27 July". The related ordinal date, covered alongside the calendar and week forms in ISO 8601 explained, counts the day of year instead. Both can be produced for any instant on the ISO 8601 formatter.
Other Week Numbering Schemes
Not every week number you meet follows the standard, and mixing schemes is the usual cause of an off-by-one dispute between two departments.
The North American Convention
Retail and broadcast calendars in the United States commonly start the week on Sunday and treat the week containing 1 January as week 1, which produces a different answer from the standard in most years. Spreadsheet functions frequently default to this behaviour, so a week number produced by a default formula should never be trusted without checking the arguments.
Broadcast and Fiscal Weeks
Retail calendars such as 4-4-5 divide a quarter into two four-week months and one five-week month, giving comparable trading periods rather than calendar accuracy. These schemes have their own year-end adjustments and occasional 53-week years, and they are not interchangeable with the standard even when the numbers happen to agree. When exchanging data across borders, state the scheme explicitly, just as you would state which of the world's date formats a numeric date is written in.
Conclusion
The ISO week number rests on two rules that between them settle every case: weeks start on Monday, and week 01 is the week holding the first Thursday of January. From those follow the 53-week years, the December days that belong to the next year's week 1, and the January days that belong to the last one. When a week number matters commercially, write it in the standard form so nobody has to guess. See the current ISO week number alongside every other representation of this moment on the local date and time page, or browse the full set of tools on localdatetime.now.