Week Number Calculator (ISO 8601) [2025]
Easily find the official ISO week number for any date. See the week's start and end dates instantly.
This calculator uses the ISO 8601 standard, common in international business and many European countries.
- Weeks start on Monday.
- Week 1 of a year is the first week containing a Thursday (or equivalently, the week containing January 4th).
- A year can have 52 or 53 weeks.
- The week number belongs to the year in which the Thursday of that week falls.
Why Use Week Numbers?
Week numbers provide a standardized way to refer to specific weeks throughout the year, which is particularly useful in business, planning, and international contexts.
Common Applications:
Project Management
Assigning tasks, tracking progress, and setting deadlines based on work weeks (e.g., "Complete by end of W32").
Scheduling & Logistics
Planning recurring events, deliveries, or production cycles on a weekly basis.
Reporting & Analysis
Aggregating data weekly (e.g., sales reports, website traffic) for trend analysis.
International Collaboration
Using a common standard (ISO 8601) avoids confusion caused by different regional calendar conventions.
Personal Planning
Organizing personal tasks, holidays, or appointments by calendar week.
Frequently Asked Questions (FAQ)
What is the ISO 8601 week numbering system?
It's an international standard defining week dates. Key features include: weeks start on Monday, Week 1 is the first week with a Thursday, and weeks belong to the year of their Thursday. This calculator adheres to that standard.
Are there other week numbering systems?
Yes, some regions (like parts of North America) might consider Sunday the start of the week, or define Week 1 simply as the week containing January 1st. This can lead to different week numbers for the same date. Using the ISO 8601 standard provides consistency.
How is the 'Year' of the week determined near New Year?
According to ISO 8601, the week number belongs to the year containing the Thursday of that week. This means dates early in January might belong to Week 52 or 53 of the *previous* year, and dates late in December might belong to Week 1 of the *next* year. Our calculator shows the correct ISO week year.
Can a year have 53 weeks?
Yes. Years that start on a Thursday, or leap years that start on a Wednesday, have 53 weeks according to the ISO 8601 standard.
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