The week number today is Week 46

Week numbers for 2026

The current week is highlighted. ISO weeks can start in December or end in January, so the first and last rows show the year when it differs. US weeks are cut at December 31 and January 1, so the first and last can be shorter than seven days.

How ISO 8601 week numbers work

ISO 8601, the international standard for writing dates and times, numbers weeks so that every week has exactly seven days and belongs to exactly one year:

• weeks begin on Monday;
• week 1 is the week that contains the year’s first Thursday, which is the same as the week containing January 4, and the first week with at least four days in the new year;
• days before week 1 belong to the last week of the previous year, and days after the last week belong to week 1 of the next.

The POSIX standard for strftime spells out the edge cases: Saturday, January 2, 1999 belongs to week 53 of 1998, and Tuesday, December 30, 1997 to week 1 of 1998 (The Open Group). A full ISO week date is written year, week, weekday: today is 2026-W46-3, where the final digit counts Monday as 1 and Sunday as 7.

Years with 53 ISO weeks

A year has 53 ISO weeks when it begins on a Thursday, or is a leap year that begins on a Wednesday. Between 2000 and 2060 these are: 2004, 2009, 2015, 2020, 2026, 2032, 2037, 2043, 2048, 2054, 2060. This year, 2026, has 53 weeks.

US week numbers

The usual American numbering, and the one Excel’s WEEKNUM uses by default, follows a different rule, which Microsoft calls System 1: weeks start on Sunday and the week containing January 1 is week 1 (Microsoft). Because the first week can be a single day, a year can have 53 or even 54 partial US weeks; 2028, a leap year starting on a Saturday, ends in week 54.

Week number formulas

Microsoft’s documentation checks the difference on March 9, 2012: ISOWEEKNUM gives 10, WEEKNUM gives 10, and WEEKNUM(…,2), Monday-start but still System 1, gives 11 (ISOWEEKNUM, WEEKNUM). Only return type 21 follows ISO 8601. Our tests check this site against the same values.

Questions about week numbers

The printable calendar shows ISO week numbers beside every month, and today’s date lists both week numbers with the other facts about today.

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