Week Number Calculator
ISO week number of a date
Use this when you need a fast, accurate result — no sign-up, works entirely in your browser.
How to Use
- Enter your details into the fields above.
- Click the calculate button to get your result instantly.
- Use the clear button to start a new calculation.
Example
- 2026-03-15 → Week 11 of ISO year 2026
- 2024-12-30 → Week 1 of ISO year 2025 (not 2024)
- 2027-01-01 → Week 53 of ISO year 2026 (not 2027)
Formula
Uses the ISO-8601 week number: every week starts on Monday, and week 1 is the week that contains the first Thursday of the year (equivalently, the week containing January 4). This is the standard used across Europe and in business/ISO contexts. The result shows both the week number and the ISO year it belongs to — these two must be read together, because the ISO year is not always the same as the calendar year: December 29–31 can already belong to week 1 of the next ISO year, and early January days can belong to week 52 or 53 of the previous ISO year. For example, 2024-12-30 is week 1 of ISO year 2025, and 2027-01-01 is week 53 of ISO year 2026.
About Week Number Calculator
ISO week number of a date. This free online tool gives you fast, accurate results without registration or downloads.
Everything runs locally in your browser, so your data is never uploaded or stored — privacy first.