Week Number Calculator: Which ISO Week Is It?
ISO weeks exist so that every week belongs to exactly one year. The rule is simple once you know it: weeks start on Monday, and week 1 holds the year's first Thursday.
What is the Week Number Calculator?
Enter a date and the tool returns its ISO week number, where weeks start on Monday and week 1 is the week containing the first Thursday of the year. That rule is why 1 January can belong to the previous year's last week, and 31 December can belong to the next year's first week.
How to use it
- Enter a date as YYYY-MM-DD.
- Click Find Week Number.
- Read the ISO week number and its ISO year.
Worked example
1 November 2026 is a Sunday in ISO week 44 of 2026, since ISO weeks start on Monday.
Why can 1 January be in week 52 or 53?
Because ISO weeks belong to the year of their Thursday. If 1 January falls before the first Thursday's week, it belongs to the previous ISO year.
Frequently asked questions
Do ISO weeks start on Monday or Sunday?
Monday, always. A Sunday belongs to the week that started the previous Monday.
How many ISO weeks can a year have?
52 or 53, depending on where the Thursdays fall.