Calendars and dates
ISO week numbers explained: why January 1 can be in week 52
Week numbers look simple until the first week of January. The ISO rule is precise, widely used in Europe and in software, and different from the convention printed on many North American calendars.
Last reviewed October 5, 2026About 3 min to readSite version v2026.10.1
The ISO 8601 rule
The international standard ISO 8601 defines weeks this way: a week starts on Monday, and week 1 of a year is the week that contains the first Thursday of that year. An equivalent statement that is easier to apply: week 1 is the week containing January 4. A third equivalent: week 1 is the first week with at least four of its days in the new year.
All three say the same thing. Because weeks never split, the days before week 1 belong to the last week of the previous year, and the days after the last full week belong to week 1 of the next year. That is how January 1, 2 and 3 can fall in week 52 or 53 of the old year, and December 29, 30 and 31 can fall in week 1 of the new year.
| January 1 is a | It belongs to | Because |
|---|---|---|
| Monday, Tuesday, Wednesday or Thursday | Week 1 of the new year | The first Thursday is in that same week |
| Friday, Saturday or Sunday | The last week of the previous year | The first Thursday is in the following week |
Years with 53 weeks
A year has 52 weeks and one extra day, or two in a leap year. Most years therefore have 52 ISO weeks. A year has 53 when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. That happens about every five to six years: 2015, 2020 and 2026 are 53-week years, and 2032 will be the next.
2026 is a useful example because it is the current year on this site: it begins on a Thursday, so January 1, 2026 is in week 1, and the year runs to week 53, which ends on Sunday January 3, 2027. The 2026 calendar prints the ISO week number beside every row, and the week number calculator gives the number for any date together with the Monday and Sunday that bound its week.
The North American convention is different
Many calendars in the United States and Canada use a different scheme: weeks start on Sunday, and week 1 is simply the week that contains January 1, however short. Under that rule the first week can be a single day long, and a year always has 53 or 54 numbered weeks.
The two schemes agree on the week number for most of the year and disagree around New Year, and sometimes by one all year long because of the Sunday versus Monday start. If a colleague says "week 14" and you are in different countries, confirm the dates. Spreadsheet software typically offers both: in most spreadsheet programs the ISO week function is separate from the default week-number function, which uses the Sunday-start convention unless told otherwise.
Writing an ISO week date
ISO 8601 writes a week date as the year, the letter W, and the two-digit week, optionally followed by a weekday number from 1 (Monday) to 7 (Sunday). 2026-W01-4 is Thursday, January 1, 2026. 2026-W53-7 is Sunday, January 3, 2027: note that the ISO week-year is 2026 even though the calendar year is 2027.
That distinction between week-year and calendar year is the source of most real-world bugs with week numbers. If you format a date as week-year plus week, do not pair it with the ordinary calendar year, or December 29, 2025 will be printed as week 1 of 2025 instead of week 1 of 2026.
See it on the site
- Week number calculatorThe ISO week for any date, with the Monday and Sunday that bound it.
- 2026 calendarEvery month with ISO week numbers down the side; a 53-week year.
- Printable calendar makerBuild a calendar that starts weeks on Monday or Sunday and shows week numbers.