ISO Week Number Finder
Instantly calculate the ISO 8601 week number for any date.
What is an ISO Week Number?
The ISO week number is part of the ISO 8601 date standard used largely in Europe, Asia, and international business. Unlike standard calendar weeks, ISO weeks have strict rules to ensure years always contain full weeks.
ISO 8601 Rules:
- ✔ Start Day: Weeks always start on Monday.
- ✔ Week 1: The first week of the year is the week containing the first Thursday of January.
- ✔ Year Length: An ISO year has either 52 or 53 full weeks.
Common Uses
- ✔ Business Planning: Supply chains and manufacturing often schedule by "Week 42" rather than specific dates.
- ✔ Fiscal Years: Accounting periods often align with full ISO weeks.
- ✔ Software Development: Project sprints are frequently tracked using ISO week numbers.