Today, Tuesday, October 6, 2026, is in week 41 of 2026 under the ISO 8601 standard. This week runs from Monday October 5 to Sunday October 11, 2026. In the US-style system used by some calendars and by Excel’s default WEEKNUM, today is week 41. Our what week is it tool shows the week number for any date.
The ISO 8601 rules
ISO 8601 is the international standard for writing dates and numbering weeks. It is used across Europe, in the UK, in many Asian countries and by most businesses that deal across borders. The rules are short:
- Weeks start on Monday and end on Sunday.
- Week 1 is the week that contains the year’s first Thursday. Put another way, it is the week containing 4 January.
- Every week belongs entirely to one year, so a year has either 52 or 53 weeks.
The Thursday rule means a week belongs to whichever year has most of its days (four or more). A side effect is that 29, 30 or 31 December can be in week 1 of the next year, and 1, 2 or 3 January can be in week 52 or 53 of the previous year.
Week 1 start dates for the coming years
| Year | Week 1 starts | Weeks in year |
|---|---|---|
| 2026 | Mon, Dec 29, 2025 | 53 |
| 2027 | Mon, Jan 4, 2027 | 52 |
| 2028 | Mon, Jan 3, 2028 | 52 |
| 2029 | Mon, Jan 1, 2029 | 52 |
| 2030 | Mon, Dec 31, 2029 | 52 |
| 2031 | Mon, Dec 30, 2030 | 52 |
| 2032 | Mon, Dec 29, 2031 | 53 |
A year has 53 ISO weeks when 1 January is a Thursday, or when it is a leap year starting on a Wednesday. 2026 has 53 weeks. More on that in how many weeks are in a year.
How US week numbers differ
In the United States, Canada and some other places, calendars usually start the week on Sunday, and week 1 is simply the week that contains 1 January, however few days of it are in the new year. That gives a few differences:
- The US system can have a partial week 1 of just one day, and can reach week 53 or even 54 in some years.
- The two systems often differ by one week number, especially early in the year.
- Calendar apps, diaries and spreadsheets may use either, so a meeting in “week 12” can mean two different weeks for people in different countries.
Many Middle Eastern calendars start the week on Saturday, and some Pakistani wall calendars start on Sunday or Monday, which again changes how a week is drawn. When in doubt, quote the date range rather than a week number.
Why week numbers are useful
Week numbers give a short, unambiguous label for a seven-day block. Factories plan production by week, schools and universities number teaching weeks, retailers compare sales week on week, and shipping lines publish sailing schedules by week. In much of Europe, people say “week 32” as naturally as a month name, while in Pakistan and the US they are mostly used at work.
How to write an ISO week
The standard form is 2026-W41 for this week, and 2026-W41-2 for today, where the last digit is the day of the week (1 = Monday, 7 = Sunday). You will see this format on shipping schedules, production plans and software release calendars.
Working out the week number yourself
Take the date’s day of the year, subtract its weekday number (Monday = 1 to Sunday = 7), add 10 and divide by 7, dropping the remainder. Today is day 279 of the year and weekday 2, so (279 − 2 + 10) ÷ 7 = 41. If the answer is 0, the date belongs to the last week of the previous year; if it is 53, check that the year really has 53 weeks.
In Excel or Google Sheets, =ISOWEEKNUM(A2) does this for you; see our spreadsheet date formulas guide. To see how many weeks remain, try weeks left in the year.
Frequently asked questions
What week number is it today?
Today, October 6, 2026, is ISO week 41. Under US-style Sunday-start numbering it is week 41.
When does week 1 start?
Under ISO 8601, week 1 starts on the Monday of the week that contains 4 January, which is also the week with the year’s first Thursday.
Do all years have 52 weeks?
No. Most years have 52 ISO weeks, but a year that starts on a Thursday, or a leap year that starts on a Wednesday, has 53.
Which countries use ISO week numbers?
ISO week numbers are standard across Europe, including the UK, and are widely used in Asia and in international business. The US and Canada more often use Sunday-start weeks.