Week Number Calculator
Ever need to know which week of the year a specific date falls into? This calculator tells you the week number according to the ISO 8601 standard. Simply pick a date, and the tool will instantly show you the corresponding week number, along with the start and end dates of that week. It's useful for project planning, scheduling, and aligning with international business calendars.
Result
49Week Number
Week Starts On
Monday, December 1
Week Ends On
Sunday, December 7
This calculation is based on the ISO 8601 standard, where weeks start on a Monday. The first week of the year is the one that contains the first Thursday of that year.
How the Week Number is Calculated
The ISO 8601 Standard
This calculator follows the international ISO 8601 standard for determining week numbers. This standard has specific rules to ensure consistency across years and countries:
- Weeks start on Monday.
- Week 1 of any year is the first week that contains a Thursday. This also means it's the first week with the majority of its days (4 or more) in that year.
- A year has either 52 or 53 weeks.
Example Calculation
For your selected date of December 1, 2025:
- The calculator identified that this date falls into the 49th week of the year, according to ISO 8601 rules.
- It also determined the start of that week (Monday) and the end of that week (Sunday), which are Dec 1 and Dec 7 respectively.