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# ISO Week Number Calculator: Date to Week and Back

Date to ISO week date and back: week 1 holds the first Thursday, weeks start Monday.
ISO 8601 week dates · weeks start Monday · week 1 holds the year’s first ThursdayResetDate → weekWeek → dateDate

today is set after the page loads in your browser

- ISO 8601-1 §7.3.6: weeks run Monday-Sunday; week 1 is the week containing the first Thursday of the year.
- An ISO year has exactly 52 or 53 weeks. Week 53 is refused for years that do not have one.
- Calendar-year ≠ ISO year at boundaries: 1 January can sit in the previous ISO year, 31 December in the next.
- All math is exact UTC-calendar arithmetic: no formatter heuristics.
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## Next door in Time

- [Timestamp converter](/time/timestamp-converter)
- [Date math](/time/date-math)
[All time tools](/time)

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Canonical HTML: https://nutter.tools/time/week-number
Markdown version: https://nutter.tools/time/week-number/index.md
Plain-text version: https://nutter.tools/time/week-number/index.txt
Agent index: https://nutter.tools/llms.txt