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What week is it?

Week 45

of 2026, ISO 8601 Monday Nov 2 – Sunday Nov 8 US week 45 2026-W45-2

Week number of any date

Week numbers by year

How week numbers work

Week numbers let people refer to a whole week with one number: “delivery in week 12”, “the office closes in week 52”. Most of Europe and much of the business world uses the ISO 8601 system, defined by the International Organization for Standardization and restated in the HTML standard that browsers implement. Its rules are short:

  1. A week starts on Monday and ends on Sunday.
  2. Week 1 is the week that contains the year’s first Thursday. The same week always contains January 4.
  3. Weeks are numbered on from there. A year has 52 weeks, or 53 if January 1 is a Thursday, or a Wednesday in a leap year.

The Thursday rule means a week belongs to the year that holds most of its days. It also means the ISO week-numbering year is not quite the calendar year: up to three days at the start of January can belong to the last week of the previous year, and up to three days at the end of December to week 1 of the next. Software that prints a week date such as 2027-W01 must use the week-numbering year, not the calendar year, or it will write 2026-W01 for the last days of December 2026.

The US system

In the United States the week usually starts on Sunday, and many calendars, spreadsheets and payroll systems call the week that contains January 1 week 1, even if that is only a single Saturday. Unicode’s locale data (CLDR), which phones and computers use for these defaults, records this as a minimum of one day in the first week, against four days for ISO countries. We found no US law or standard that defines it, so treat it as a convention. The two systems give the same number for much of the year in some years and differ by one in others, which is why we always show both.

Other conventions

In Egypt, Iran, Iraq, Jordan, Kuwait, Oman, Qatar and several other countries the printed week starts on Saturday. Week numbers are rarely used there in everyday life; when they are, software generally follows ISO 8601 or the local Saturday-start rule set on the device.

Where January 1 falls, year by year

YearJan 1Jan 1 is inWeek 1 startsDec 31 is inISO weeks
2024MondayW01January 1W01 of 202552
2025WednesdayW01December 30, 2024W01 of 202652
2026ThursdayW01December 29, 2025W5353
2027FridayW53 of 2026January 4W5252
2028SaturdayW52 of 2027January 3W5252
2029MondayW01January 1W01 of 203052
2030TuesdayW01December 31, 2029W01 of 203152
2031WednesdayW01December 30, 2030W01 of 203252
2032ThursdayW01December 29, 2031W5353

Common mistakes

  • Using the calendar year with the ISO week. December 29, 2025 is in week 1 of 2026, not week 1 of 2025.
  • Assuming every year has 52 weeks. Plans and payroll cycles break in 53-week years such as 2026 and 2032.
  • Mixing systems. A spreadsheet’s WEEKNUM function counts US style by default; ISOWEEKNUM (Excel and Google Sheets) gives the ISO number.
  • Counting from the first Monday. If January 1 is a Tuesday, Wednesday or Thursday, week 1 starts in December of the previous year.

The calendar can show ISO or US week numbers next to every month, and the printable calendar prints them.

Week numbers in spreadsheets and code

Excel and Google Sheets

  • =ISOWEEKNUM(A1) gives the ISO 8601 week of the date in A1. For December 28, 2026 it returns 53.
  • =WEEKNUM(A1) uses Microsoft’s “System 1”: the week containing January 1 is week 1 and weeks start on Sunday, the US count. A second argument changes the start day (2 = Monday) but keeps System 1.
  • =WEEKNUM(A1, 21) is “System 2”, the ISO rule, the same as ISOWEEKNUM.
  • The ISO week-numbering year is not =YEAR(A1) around New Year. One formula that gives it is =YEAR(A1-WEEKDAY(A1,2)+4), the year of that week’s Thursday.

Python

  • date(2026, 12, 28).isocalendar() returns (2026, 53, 1): ISO year, week and weekday (Monday = 1).
  • date.fromisocalendar(2027, 1, 1) goes the other way: Monday of week 1 of 2027, January 4.
  • In strftime, %V is the ISO week and %G its year. %U is not the US count: it puts the days before the first Sunday in week 0.

Each week page, such as week 53 of 2026, lists its seven dates with the ISO week date and the US week number of every day.

Questions

What week is it?
The week number at the top of this page is the current ISO 8601 week for your time zone, with its Monday-to-Sunday dates and the US week number beside it. ISO weeks start on Monday; US weeks start on Sunday and count the week of January 1 as week 1.
How are ISO week numbers counted?
Weeks run from Monday to Sunday. Week 1 of a year is the week that contains the first Thursday of January; equivalently, the week that contains January 4, or the first week with at least four days in the new year. Every later week is numbered in order, so a year ends with week 52 or 53.
Why does January 1 sometimes belong to week 52 or 53?
Because ISO weeks are never split between years. If January 1 falls on a Friday, Saturday or Sunday, that week has more days in the old year than in the new one, so it is the last week of the old year. January 1, 2027 is a Friday and belongs to week 53 of 2026; week 1 of 2027 begins on Monday, January 4.
Which years have 53 weeks?
A year has 53 ISO weeks when January 1 is a Thursday, or a Wednesday in a leap year. 2020 and 2026 are 53-week years; the next one is 2032. Week 53 of 2026 runs from December 28, 2026 to January 3, 2027.
Is the US week number different?
Often by one. US calendars start the week on Sunday and count the week that contains January 1 as week 1, however few days of it are in January. That rule is a custom recorded in Unicode’s locale data, not a law. Both numbers for the current week are shown at the top of this page.
What is an ISO week date?
A date written as year, week and weekday: 2026-W45-1 is Monday of week 45 of 2026. The year in it is the ISO week-numbering year, which differs from the calendar year for a few days around New Year: December 29, 2025 is 2026-W01-1.

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