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ISO Week Number Finder

Determine the ISO-8601 week number for any calendar date. Ideal for project planning, reporting, and international collaboration.

Free to use, no watermarks, runs in your browser—nothing uploads to our servers

Date

The calculator follows ISO-8601, where weeks start on Monday and week 01 is the week containing the first Thursday of the year.

Some late-December dates belong to week 01 of the next ISO year. This occurs when the week extends into January.

Week numbers are valuable for international project planning, production schedules, and aligning with enterprise reporting cycles.

Report a bug

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