Week Number Calculator
Find the ISO week number for any date
View All Weeks in 2026
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ISO Week Number Calculator
Quickly find the ISO 8601 week number for any date in past or future years. While standard calendars divide the year into months, many businesses, governments, and manufacturing industries plan their operations using week numbers (W01–W53).
Understanding ISO 8601
The International Organization for Standardization (ISO) sets the global standard for week numbering:
- Week 1: The week containing the first Thursday of the year (or Jan 4th).
- Start Day: Weeks always begin on Monday.
- Duration: Years have 52 or 53 weeks (leap weeks).
Regional Differences
Industry Use Cases
Manufacturing
"Production scheduled for W42." Essential for lead times and supply train tracking.
Fiscal Planning
Quarterly reports are often divided into 13-week blocks.
Agile Sprints
Development teams often number sprints by the calendar week.
Education
Universities often schedule semesters by teaching weeks.