1. How the week number is calculated
We follow the ISO 8601 international standard:
- Weeks run from Monday to Sunday;
- The week containing the year's first Thursday is week 1;
- Most years have 52 weeks; 53 weeks occur when 1 Jan is Thu or, in leap years, Wed.
Therefore 3 Dec 2025 falls in week 49 of 2025, ranging from 1 Dec to 7 Dec.
2. What is the ISO 8601 week numbering?
ISO 8601 is the global standard for date and time representation, used in finance, education, logistics, project management, etc. Its week rules:
- Monday = day 1, Sunday = day 7;
- Year boundaries may place dates into the last week of the previous year or the first week of the next;
- Weeks are noted with a “W” and two digits, e.g. 2025-W49-03 (the 3rd day of week 49, 2025).
Use our tool to convert any date into the correct ISO week and share the result via URL—perfect for rosters, timetables, payroll and sprint planning.
3. Typical use-cases
Work / Rosters / Time-cards
- • Which work week is it?
- • Shift-schedule week index
- • Payroll week validation
Education / Semesters
- • Academic teaching week
- • School-term week counter
- • Exam-week planning
Finance / Accounting
- • Fiscal-week alignment
- • Period-close checks
- • Weekly KPI reporting
Project & Sprint Management
- • Sprint week index
- • Agile iteration planning
- • Milestone week tracking
4. FAQ
Most years have 52 weeks. If 1 January is a Thursday—or a Wednesday in a leap year—the year will have 53 weeks. Our calculator shows the correct count automatically.
ISO 8601 defines Monday as the first day of the week and Sunday as the seventh. The week containing the year’s first Thursday is week 1.
Simply copy the URL—every result page includes a date parameter and is SEO-friendly, so search engines can display rich snippets.