Week Number Calculator
Choose a date to see which week of the year it belongs to. The main result follows the international ISO 8601 standard, with the US-style week number shown alongside.
Type a number and the answer appears straight away.
The math behind it: Week Number
ISO week 1 is the week, starting on Monday, that contains the first Thursday of the year
Under ISO 8601, weeks run from Monday to Sunday and are numbered from the week that holds the year's first Thursday. November 6, 2026 falls in week 45. Most years have 52 weeks, and some, including 2026, have 53.
Common questions
How do I find the week number of a date?
Enter the date above. By hand, find the Monday of the week containing January 4, which starts week 1, and count the weeks from there.
How many weeks are in a year?
52 weeks and one day, or two days in a leap year. In the ISO system most years have 52 numbered weeks and about one in six has 53.
Why do the US and Europe number weeks differently?
Europe follows ISO 8601, where weeks start on Monday and week 1 contains the first Thursday. The common US convention starts weeks on Sunday and calls the week containing January 1 week 1, so the numbers can differ by one.
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