How to use the week number tool
The page opens on today's date, so the big number at the top is this week's ISO week number. Pick any other date to see its week. The table lists every ISO week of the year with its Monday and Sunday, and highlights this week. Use the arrows or type a year to see the weeks of another year.
The result also shows the ISO week date, such as 2026-W41-3 for Wednesday, October 7, 2026: the week-numbering year, the week, then the day of the week counted from Monday as 1 to Sunday as 7.
How ISO 8601 week numbers work
ISO 8601 is the international standard for writing dates. Its week numbers follow two rules:
- Weeks start on Monday and end on Sunday.
- Week 1 is the week with the year's first Thursday in it. That is the same as saying week 1 is the week that contains January 4, or the first week with at least four days in the new year.
Every week belongs wholly to one year, the year its Thursday falls in. So the last days of December can be in week 1 of the next year, and the first days of January can be in week 52 or 53 of the year before.
| Date | ISO week | Why |
|---|---|---|
| Mon, Dec 29, 2025 | 2026, week 1 | Its Thursday is January 1, 2026 |
| Wed, Oct 7, 2026 | 2026, week 41 | The 41st Monday-to-Sunday week |
| Thu, Dec 31, 2026 | 2026, week 53 | 2026 starts on a Thursday |
| Fri, Jan 1, 2027 | 2026, week 53 | Its Thursday is December 31, 2026 |
| Mon, Jan 4, 2027 | 2027, week 1 | The week with January 4 in it |
Years with 53 weeks
Most years 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, because then it has 53 Thursdays. 2026 starts on a Thursday, so it is a 53-week year: week 53 runs from Monday, December 28, 2026 to Sunday, January 3, 2027. The years with 53 weeks from 2000 to 2040 are 2004, 2009, 2015, 2020, 2026, 2032 and 2037.
Leap years follow the Gregorian rule, as the US Naval Observatory puts it: "Years evenly divisible by 4 are leap years, with the exception of centurial years that are not evenly divisible by 400."
US week numbers (weeks starting Sunday)
Many US calendars and spreadsheets number weeks differently: weeks start on Sunday, and week 1 is simply the week with January 1 in it, however few days of it are in January. This is the default in Excel's WEEKNUM function, which Microsoft describes as "System 1":
"The week containing January 1 is the first week of the year, and is numbered week 1."Microsoft Support, WEEKNUM function
Excel's "System 2" is the ISO rule; Microsoft notes it "is the methodology specified in ISO 8601". With the US system the week number never moves into another year, so the first and last weeks are often short, and a year can reach week 53, or even 54 in a leap year that starts on a Saturday, such as 2028.
The two systems often agree, as on October 7, 2026, which is week 41 in both. They differ around New Year and on Sundays: Sunday, January 4, 2026 is in ISO week 1 but US week 2, and Friday, January 1, 2027 is in ISO week 53 of 2026 but US week 1 of 2027. If someone gives you a week number, check which system they mean. Microsoft calls the ISO system the one "commonly known as the European week numbering system".
Questions and answers
What week number is it today?
The number at the top of this page is today's ISO week number, worked out from your device's date when the page opens. The US week number, counting from Sunday, is shown under it.
Does 2026 have 53 weeks?
Yes. 2026 starts on a Thursday, so it has 53 ISO weeks. Week 53 runs from Monday, December 28, 2026 to Sunday, January 3, 2027, and week 1 of 2027 starts on Monday, January 4, 2027.
When does week 1 of 2026 start?
ISO week 1 of 2026 starts on Monday, December 29, 2025 and ends on Sunday, January 4, 2026, because it contains the first Thursday of 2026, January 1.
Why does my calendar show a different week number?
Your calendar probably starts weeks on Sunday and counts the week with January 1 as week 1, as many US calendars and Excel's WEEKNUM do by default. This page shows both numbers so you can compare.
How do I get the ISO week number in Excel?
Use ISOWEEKNUM(date), or WEEKNUM(date, 21). Plain WEEKNUM(date) uses the Sunday-start system instead.
Sources
Checked on 7 October 2026. If a rule has changed, please tell us.