ISO week number

ISO-8601 defines week 1 as the week containing the first Thursday of the calendar year. Dates in early January that fall before that Thursday belong to week 52 or 53 of the previous year, and dates in late December that fall after the last Thursday may belong to week 1 of the next year.

A year has 53 ISO weeks if 1 January falls on a Thursday, or if it is a leap year and 1 January falls on a Wednesday or Thursday. This occurs roughly 5 or 6 times per decade.

Yes. Weeks start on Monday, the minimum days-in-week required for week 1 is 4 (the first-Thursday rule), and the week number runs from 01 to 52 or 53. This matches the standard used in most European business scheduling systems.

No. The ISO week calculation is computed from the date integers using a standard algorithm in the browser. The date you look up is not sent anywhere; it stays within the browser tab for the duration of your session.

Yes. The US convention (used in some spreadsheet applications) counts weeks starting from Sunday, and week 1 begins on 1 January regardless of what day that is. This tool implements the ISO standard exclusively. If you need US-style week numbers, check your spreadsheet's WEEKNUM function with the second argument set to 1.

Yes. The page includes a reference table listing every ISO week of the chosen year (52 or 53 rows) with the Monday to Sunday date range of each week. It shows the current year by default and you can switch to any other year.