ISO Week Number Calculator
Find the ISO 8601 week number and ISO year for any date.
Quick answer: Find the ISO 8601 week number and ISO year for any date.
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Frequently asked questions
- Is the week number calculator free?
- Yes — free with no signup.
- Why is the ISO year sometimes different from the calendar year?
- Days early in January can belong to week 52/53 of the previous ISO year, and vice versa for late December.
- Does it match what Outlook and Google Calendar show?
- Yes — both default to ISO 8601 week numbering in EU locales.
- Are my dates uploaded?
- No — every calculation runs in your browser.
- What about US-style week numbering?
- US numbering starts weeks on Sunday and uses different rules. This tool is ISO-only.