Week Number Calculator
The ISO week number for any date.
How ISO week numbers work
Weeks run Monday to Sunday, and week 1 is the week containing the year's first Thursday — equivalently, the week containing 4 January. This means the last few days of December can belong to week 1 of the following year, and the first days of January can belong to the last week of the previous year.
Example: 29 December 2025 (a Monday) is actually in week 1 of 2026, not week 53 of 2025.