Week Calculator
Find ISO 8601 week numbers, week start and end dates for any calendar year.
Monday, Sep 7, 2026 to Sunday, Sep 13, 2026
About the Week Calculator
Under the international ISO 8601 standard, weeks begin on Monday and are numbered from 1 to 52 (or 53 in leap-week years). The Week Calculator identifies the official ISO week number for any date and reveals the corresponding start and end dates for any week in history.
How to Use This Calculator
- Enter any date to find its ISO Week Number.
- Alternatively, enter a Year and Week Number (e.g., Week 38 of 2026) to generate the exact Monday-to-Sunday date range.
- View total week count and leap-week classifications.
Calculation Method & Mathematical Formula
According to ISO 8601, Week 01 of any year is defined as the week containing the first Thursday of that year (or equivalently, the week containing January 4th).
Worked Example
Scenario: Checking September 13, 2026:
- ISO Week Number: Week 37 of 2026.
- Week Start (Monday): September 7, 2026.
- Week End (Sunday): September 13, 2026.
- Total Weeks in 2026: 53 Weeks (Leap Week Year).
Important Calendar Notes
In the United States and Canada, traditional calendars often consider Sunday as the first day of the week. ISO 8601 strictly defines Monday as Day 1 for international consistency.