What Week Number Is It? ISO Week Numbers Explained

On Friday, October 2, 2026, it is ISO week 40 of 2026, the week from Monday, September 28 to Sunday, October 4. Under the ISO 8601 standard, weeks start on Monday and week 1 is the week that contains the year’s first Thursday.

Week numbers look simple until you compare two calendars and find they disagree by one. That happens because there are two common systems: the international ISO 8601 rule, which Microsoft’s Excel documentation calls the European system, and the US habit of starting week 1 on January 1 with Sunday as the first day. This guide explains the ISO rules, shows where the two systems split, lists the years with 53 weeks and gives the Excel formulas for each. For today’s number at any time, open the current week number page.

The ISO 8601 week rules

• Weeks run Monday to Sunday. Every ISO week has exactly seven days, with no partial weeks.
• Week 1 contains the first Thursday of January. Put another way, week 1 is the first week with at least four days in the new year, and it always contains January 4.
• A year has 52 or 53 weeks. The last week is the one that contains the year’s last Thursday.
• The week belongs to the year of its Thursday. So up to three days at the start of January can sit in the last week of the previous year, and up to three days at the end of December can sit in week 1 of the next year.

The calendar around New Year 2027 shows the last rule at work. January 1, 2027 is a Friday, so Friday January 1 to Sunday January 3, 2027 are part of week 53 of 2026. ISO week 1 of 2027 starts on Monday, January 4, 2027. A year earlier the opposite happened: January 1, 2026 was a Thursday, so week 1 of 2026 began on Monday, December 29, 2025.

When week 1 starts: 2026 to 2032

See every week with its Monday-to-Sunday dates in the 2027 week number calendar or the 2026 week numbers.

Which years have 53 weeks?

A year has 53 ISO weeks when January 1 is a Thursday, or when it’s a leap year and January 1 is a Wednesday. Either way the year contains 53 Thursdays. Between 2010 and 2047 that happens in 2015, 2020, 2026, 2032, 2037 and 2043, so a 53-week year comes around every five or six years.

2026 is one of them. ISO week 53 of 2026 runs from Monday, December 28, 2026 to Sunday, January 3, 2027. The next 53-week year after that is 2032. If you plan by week number, a 53rd week can surprise budgets, rota templates and reports that assume 52 weeks; our guide to how many weeks are in a year covers what that means for pay and planning.

ISO vs US week numbers

The US convention, which is also the default in Excel, numbers the week that contains January 1 as week 1 and starts each week on Sunday. The first and last weeks can be partial, so a year can have 53 or even 54 numbered weeks. Most of the year the two systems give the same number, but they split on Sundays and around New Year:

Two practical rules follow. First, when you share a week number with someone in another country or company, say which system you mean, or give the Monday date as well. Second, never pair an ISO week with the calendar year without checking: “week 53, 2026” includes three days of January 2027.

Week numbers in Excel

Excel has a function for each system. Microsoft documents them on its WEEKNUM and ISOWEEKNUM support pages.

• =ISOWEEKNUM(A1) returns the ISO 8601 week. =ISOWEEKNUM(DATE(2027,1,1)) returns 53.
• =WEEKNUM(A1) uses the default return type 1: the week containing January 1 is week 1, and weeks begin on Sunday. =WEEKNUM(DATE(2027,1,1)) returns 1.
• =WEEKNUM(A1,2) keeps the January 1 rule but starts weeks on Monday.
• =WEEKNUM(A1,21) uses the ISO system, the one Microsoft calls System 2, and matches ISOWEEKNUM.

Microsoft’s own example: for March 9, 2012, ISOWEEKNUM returns 10, WEEKNUM returns 10 and WEEKNUM with return type 2 returns 11. If a spreadsheet and a calendar app disagree, check which return type the formula uses before anything else.

Finding the week number by hand

To work out an ISO week without software, find the Thursday of the week your date is in (the Monday-to-Sunday week). Count which Thursday of its year that is: the first Thursday is in week 1, the second in week 2, and so on. For Friday, October 2, 2026, the Thursday of that week is October 1, 2026, the 274th day of the year. Divide the day of the year by 7 and round up: 274 ÷ 7 is 39.1, which rounds up to 40, so October 1 is the 40th Thursday and the date is in week 40. This shortcut works because the first Thursday always falls on day 1 to 7. The same method explains the New Year cases: the Thursday of the week containing January 1, 2027 is December 31, 2026, so that week belongs to 2026.

A printed 2027 calendar with week numbers saves the arithmetic for the year ahead.

Frequently asked questions

What week number is it today?

On Friday, October 2, 2026 it’s ISO week 40, which runs from Monday, September 28 to Sunday, October 4, 2026. The week number page always shows the current week.

Does 2026 have 53 weeks?

Yes. 2026 starts on a Thursday, so it has 53 ISO weeks. Week 53 runs from December 28, 2026 to January 3, 2027. 2027 has 52 weeks.

Does the week start on Sunday or Monday?

Under ISO 8601 it starts on Monday. US calendars and Excel’s default WEEKNUM start on Sunday, which is why the two can differ on Sundays.

Why is January 1 sometimes in week 52 or 53?

ISO weeks belong to the year of their Thursday. If January 1 falls on a Friday, Saturday or Sunday, that week’s Thursday is in December, so the days belong to the last week of the old year.

What’s the difference between WEEKNUM and ISOWEEKNUM?

WEEKNUM, by default, counts from the week containing January 1 with Sunday-start weeks. ISOWEEKNUM follows ISO 8601: Monday-start weeks, with week 1 containing the first Thursday.