How to use the Week Number Calculator
Enter the date and the result appears straight away. Everything runs in your browser, so nothing you type is uploaded.
How it works
ISO 8601: weeks start on Monday and week 1 contains the first Thursday.
Examples
- 10 November 2026
ISO week 46 of 2026 Day of the year: 314 Week runs Mon, November 9, 2026 to Sun, November 15, 2026
- 1 January 2026
ISO week 1 of 2026 Day of the year: 1 Week runs Mon, December 29, 2025 to Sun, January 4, 2026
- 31 December 2026
ISO week 53 of 2026 Day of the year: 365 Week runs Mon, December 28, 2026 to Sun, January 3, 2027
Limitations and assumptions
ISO weeks. US calendars that start weeks on Sunday can number weeks differently.
Frequently asked questions
How many weeks are in a year?
An ISO year has 52 or 53 weeks.
Why can 1 January be in week 52 or 53?
Early January days can belong to the last ISO week of the previous year.
Do all countries number weeks the same way?
No. The US often starts weeks on Sunday, so numbers can differ from ISO.
How do I count weeks between dates?
Use the weeks between dates calculator.