What week is it?

The box at the top shows the current ISO week number for your time zone, the dates of its Monday and Sunday, and the US-style week number for comparison.

52 weeks and one day (two in a leap year). In ISO numbering a year has either 52 or 53 weeks: it has 53 when it starts on a Thursday, or is a leap year starting on a Wednesday. 2026 and 2032 have 53 ISO weeks; 2027 has 52.

Yes. If 1 January is a Friday, Saturday or Sunday, it belongs to the last ISO week of the previous year. For example, 1 January 2027 is in week 53 of 2026. Likewise 29–31 December can be in week 1 of the following year.

Use =ISOWEEKNUM(A1) for the ISO week. =WEEKNUM(A1) on its own gives the US-style number with Sunday-start weeks; =WEEKNUM(A1,21) is the ISO number in Excel.

Python: date.today().isocalendar().week. PostgreSQL: extract(week from now()) (ISO). In JavaScript there is no built-in ISO week function yet; Temporal's PlainDate.weekOfYear will provide one, and until then you need a few lines of arithmetic.

Most of Europe, and international standards, do. The United States, Canada and Japan commonly start weeks on Sunday, and some Middle Eastern countries on Saturday. When a document gives a week number without saying which system, check one known date.