“ISO week numbers are the international standard for weekly planning – used in business, logistics, and education worldwide.”
Understanding ISO Week Numbers
Week numbers are essential for project planning, academic calendars, and business reporting. Our calculator gives you the ISO week number and the exact Monday‑to‑Sunday range for any date.
How to Use
- Pick a date (or use today).
- Instantly see the ISO week number and the week's start/end dates.
- The calendar highlights the selected date and the entire week.
ISO 8601 Rules
- Weeks start on Monday.
- Week 1 is the week containing the first Thursday of the year.
- Year boundaries may shift: e.g., December 31 can belong to week 1 of next year.
Common Use Cases
- Business planning – align deadlines with week numbers.
- Education – track academic weeks.
- Project management – schedule tasks by week.
Frequently Asked Questions
Why does the week number sometimes change on January 1?
Because week 1 is defined by the first Thursday. If Jan 1 is a Friday, it belongs to the last week of the previous year.
Can I use this for fiscal years?
Yes, but fiscal years may start at different times. The tool uses the standard calendar year.
What is the range of week numbers?
ISO weeks range from 1 to 53, with 53 occurring in years that have 53 Thursdays.