Week Number Calculator
Find the ISO week number of a date and its bounds (Monday to Sunday).
Default: today.
How is this result calculated?
The ISO week runs from Monday to Sunday. To find the week number of a date, the Thursday of the calendar week containing it is determined first: the year of that Thursday gives the "ISO year", which can differ from the calendar year of the original date at the very start or end of a year.
The week number is then the number of Thursdays elapsed since January 1 of that ISO year, plus 1, i.e.: Math.ceil((days between January 1 and that Thursday + 1) / 7). The Monday and Sunday shown are simply the bounds of the week containing that Thursday.
Frequently asked questions
Why does the ISO 8601 standard start the week on Monday?+
The international ISO 8601 standard sets Monday as the first day of the week (and Sunday as the last), unlike other conventions that start on Sunday. This removes ambiguity in international exchanges, particularly for scheduling and invoicing.
Why can the ISO year differ from the calendar year?+
An ISO week can never be split across two different years: it belongs entirely to the calendar year of its Thursday. As a result, the very first days of January can belong to the last week of the previous year (e.g. January 1, 2023 falls in week 52 of 2022), and the last days of December can belong to the first week of the following year (e.g. December 31, 2024 falls in week 1 of 2025).
Does a year always have 52 weeks?+
Most calendar years have 52 ISO weeks, but some have 53: this happens when January 1 falls on a Thursday, or on a Wednesday in a leap year. These 53-week years occur roughly once every 5 to 6 years.
How is this different from the "US" week number?+
Some conventions, notably in the United States, start the week on Sunday and number weeks differently. This calculator strictly follows the ISO 8601 standard (Monday-to-Sunday week) and does not compute the "US" numbering.
What is the ISO week number used for in practice?+
It's widely used for project planning, team schedules, invoicing, production tracking, and weekly activity reports, especially in Europe where it's the reference convention.