Week Number Calculator

Find the ISO week number for any date. Determine what week of the year a date falls in.

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ISO Week Number

The ISO 8601 standard defines week 1 as the week containing the first Thursday of the year. Weeks start on Monday. A year has either 52 or 53 weeks.

Example: January 15, 2025 → Week 3

Last updated: 2026-08-08