Week Number Calculator

Find the ISO week number for any date

Enter a date to get its ISO-8601 week number, the ISO year that week belongs to, and the weekday name. The week number defaults to today so you can see the current week at a glance. ISO-8601 weeks start on Monday, and week 1 is defined as the week containing the first Thursday of the year (equivalently, January 4). All arithmetic is done in UTC so the result is the same regardless of your local timezone.

Note that the ISO year can differ from the calendar year for the first few or last few days of January and December. For example, January 1 sometimes belongs to week 52 or 53 of the previous year, and December 31 can belong to week 1 of the following year. The tool reports the correct ISO year alongside the week number so there is no ambiguity.

FAQ

Why does the ISO year differ from the calendar year? ISO weeks always start on Monday and end on Sunday. A partial week at the start of January that contains fewer than four days is considered the last week of the previous year.

Can a year have 53 weeks? Yes. A year has 53 ISO weeks if January 1 falls on Thursday, or if it is a leap year and January 1 falls on Wednesday.

What does the weekday number mean? ISO weekdays are numbered 1 (Monday) through 7 (Sunday), which is different from JavaScript's getDay() which uses 0 for Sunday and 6 for Saturday.