Week Number Calculator
Enter a date to find its ISO week number and ISO week year.
| 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.
How it works
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
When to use this tool
This tool is designed for quick, practical tasks such as everyday calculations, data formatting, or simple conversions. It is best used when you need fast results without installing software or using complex tools.
When to use
- Quick checks or one-time calculations
- Validating or converting data before using it elsewhere
- Simple tasks that do not require advanced software
When not to use
- Critical financial, legal, or medical decisions
- Large-scale or automated processing
- Situations requiring guaranteed precision beyond basic validation
Always review results before using them in important contexts.
About this tool
This tool helps you perform quick utility operations directly in your browser. It runs entirely in your browser without sending data to a server.
You can use this tool when handling simple tasks without installing additional software. The results should be interpreted as a processed output based on your input data.
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.