Week Number Calculator
ISO week number, week-year and week range for any date.
ISO 8601 weeks start Monday and week 1 is the one containing the first Thursday of the year — so dates near New Year can belong to week 52/53 of the previous year. The US convention (Sunday-start, Jan 1 = week 1) differs; ISO is what most calendars in Europe and APIs use.
How to use the Week Number tool
- Enter your week number in the fields above.
- Adjust any slider or type an exact number — the answer updates as you go.
- Copy the result and close the tab. Nothing you entered was sent anywhere.
About the Week Number tool
Get the ISO-8601 week number and week-year for any date, the exact Monday-to-Sunday range of that week, and its named weekday.
The week-year matters at New Year: Jan 1 can fall in week 52 of the previous ISO year, which quietly breaks naïve year+week grouping in reports.
Everything runs locally in your browser: no data is uploaded, no account is needed, and it works offline once the page has loaded.
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