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ISO Week Number Calculator

Find the ISO 8601 week number, week-based year, and Monday-to-Sunday range for any date.

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Result

ISO week dates are common in manufacturing, logistics, finance, and sprint planning. Weeks begin on Monday, and week 1 is the week containing the year’s first Thursday.

Because of that rule, the first days of January can belong to the previous ISO week-year, while late December can belong to the next one.

Choose any date to see its ISO week number, week-based year, and the exact Monday-to-Sunday range. The algorithm follows ISO 8601 rather than locale-specific calendar settings.

The result is calculated instantly in the browser.

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Accepted Formats & Rules

  • ISO week number
  • ISO week-based year
  • Monday and Sunday boundaries

Key Benefits

✓ Uses the international ISO 8601 week standard.
✓ Correctly handles dates near New Year that belong to another week-year.
✓ Shows the full week range alongside the number.

Use Cases

  • • Looking up the ISO week number used in a sprint or production plan?
  • • Checking which week-year a date near New Year belongs to?
  • • Need the Monday and Sunday boundaries of a numbered week?