Week Number Calculator
Enter any calendar date and see its ISO week number. This tool follows the ISO-8601 standard, where weeks start on Monday and week 1 is the week with the year’s first Thursday.
ISO Week for Any Date
The date format depends on your browser and region settings (for example DD.MM.YYYY or MM/DD/YYYY).
Quick Week Facts
Today
Week Number Highlights for This Year
All values are calculated based on the ISO-8601 week date system and today’s date in your local time zone.
How this week number calculator works
This calculator shows the ISO week number for any given date. Simply select a date and the tool applies the ISO-8601 rules to determine the correct calendar week. In this system, weeks start on Monday and week 1 is the week that contains the first Thursday of the year.
This makes week numbers consistent across countries and calendar systems that follow the ISO standard. The result is especially useful for project planning, reporting, payroll periods and many business contexts where weeks are used as a time unit.
The displayed date format is determined by your browser and regional settings (for example DD.MM.YYYY or MM/DD/YYYY). The ISO week calculation is based solely on the selected calendar date and is not affected by how the date is visually formatted.
Typical uses for a week number calculator
- Project planning: Align tasks, deadlines and sprints with specific ISO calendar weeks.
- Business reporting: Group sales, traffic or performance data by ISO weeks instead of months.
- HR and payroll: Check which week a pay period, shift rotation or timesheet entry belongs to.
- School and academic schedules: Map lessons, exams and holidays to week numbers.
- Personal planning: Coordinate vacations, appointments or training plans using weeks as a reference.
Related Calculators
If you work with ISO weeks and calendar dates, you may also want to calculate the number of days between two dates with our Days Between Dates Calculator, count how many days have passed since an event using the Days Since Date Calculator, or add and subtract days, weeks, months, or years with the Add or Subtract Date Calculator. To check weekdays for planning, the Weekday Calculator is a helpful companion tool.
These tools are often used together for scheduling, reporting, and planning across calendar weeks and date ranges.
- How many days until my next vacation?
- How many days are left until a deadline?
- Measure the duration between a project start and end date.
- How many days since I started my new job?
- How many days have passed since January 1st, 2020?
- Track days since an important event.
- What date is 30 days from today?
- Subtract 45 days from a deadline.
- Plan follow-up dates and schedules.
- What weekday is my meeting date?
- What day of the week was I born?
- Check weekdays for travel planning.
- How many hours and minutes did I work today?
- What is the time difference between 08:30 AM and 05:45 PM?
FAQ
What is an ISO week number?
An ISO week number follows the ISO-8601 standard. Weeks start on Monday, and week 1 is the week that contains January 4 or, equivalently, the first Thursday of the year. This can mean that some dates at the beginning or end of a calendar year actually belong to the previous or next ISO week year.
Why does the week year sometimes differ from the calendar year?
Because ISO weeks are defined by the position of Thursdays, dates in early January or late December can belong to the previous or next ISO week year. For example, January 1 might still be part of the last ISO week of the previous year.
Which years are supported?
The calculator is designed to work with a wide range of years, typically from year 1 up to year 9999, depending on what your browser allows for the date input. There is no artificial minimum such as “from 1900 only”.
Why do I see a different date format?
The date picker follows your browser’s regional settings. Some regions use day–month–year, others month–day–year. The internal week number calculation is always correct, regardless of the visual format.
Do time zones or daylight saving time matter?
No. Week numbers are based on whole calendar days and the ISO-8601 week date system. Time zones and daylight saving switches do not change the ISO week number of a given date.