Week Number Calculator
Enter a date to find its ISO week number and ISO week year.
Week Number Calculator tool
| Metric | Value |
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This tool uses ISO 8601 week numbering: Monday is the first day of the week.
Privacy: calculations run locally in your browser. No dates are stored or transmitted.
ISO week numbering assigns dates to week-based years. Week 1 is the week containing the first Thursday of the year. This means some dates near January 1 or December 31 can belong to the previous or next ISO week year.
Examples
- 2026-01-01 may not always be in ISO week 1 depending on the calendar alignment
- Dates in late December can belong to week 1 of the next ISO year
- Useful for project schedules, reporting, and international planning
FAQ
- What is a week number?
A week number identifies which week of the year a date belongs to. This calculator uses ISO week numbering.
- What is ISO week numbering?
ISO week numbering starts weeks on Monday, and week 1 is the week that contains the first Thursday of the year.
- Why can early January belong to the previous ISO year?
Because ISO week years are based on full weeks, some dates near the start or end of a calendar year belong to a neighboring ISO week year.
- Does this work for leap years?
Yes. The calculation works with real calendar dates, including leap years.
- Are my dates stored?
No. Everything runs locally in your browser.