Use the week number calculator to find the ISO week number for any date.
Choose a date below to see the week number, ISO week year, weekday and the Monday-to-Sunday date range for that week.
Find the week number for any date
The calculator above uses the ISO 8601 week-numbering system.
Under this system:
- Weeks start on Monday.
- Weeks end on Sunday.
- Week 1 is the week containing January 4.
- Week 1 is also the week containing the first Thursday of the year.
- A year can contain 52 or 53 ISO weeks.
How to use the week number calculator
- Choose a date.
- Select Find week number.
- See the ISO week number and week year.
- View the Monday and Sunday dates for that week.
The result also shows the weekday and day of year.
What is an ISO week number?
An ISO week number identifies a Monday-to-Sunday period within an ISO week-numbering year.
For example, a date may belong to:
Week 35 of 2026
even though the exact calendar date is different.
Week numbers are commonly used in:
- Business planning
- Logistics
- Production schedules
- Project planning
- School calendars
- Delivery planning
- Reporting periods
Why can the ISO week year differ from the calendar year?
Dates near New Year can belong to a different ISO week-numbering year.
For example, the first few days of January may still belong to week 52 or 53 of the previous ISO year.
Likewise, the final days of December can sometimes belong to week 1 of the next ISO year.
This is why the calculator shows both the week number and ISO week year.
When does an ISO week start?
An ISO week starts on Monday and ends on Sunday.
The calculator displays the exact Monday and Sunday for the selected date.
How many ISO weeks are in a year?
Most years contain 52 ISO weeks, but some years have 53.
The exact number depends on how the calendar aligns with the ISO week-numbering rules.
Week numbers in the United States
Many US calendars visually begin with Sunday, while ISO weeks begin with Monday.
The calculator uses ISO week numbers regardless of the calendar layout commonly used in a particular country.
If you are sharing a week number internationally, it is useful to specify that you mean an ISO week number.
Need the current week number?
If you only want to know what week it is right now, use the Current Week Number page.
Related date tools
You may also find these useful:
- Current Week Number
- Calendar
- Date Calculator
- Days Between Dates
- Business Days Calculator
- Today’s Date
Frequently asked questions
What week number is a date?
Enter the date in the calculator above to see its ISO week number.
Does the calculator use ISO week numbers?
Yes. It follows the ISO 8601 week-numbering system.
Does an ISO week start on Monday?
Yes. ISO weeks run from Monday through Sunday.
Can January 1 belong to the previous ISO year?
Yes. January 1 can belong to week 52 or 53 of the previous ISO week-numbering year.
Can December belong to week 1 of the next year?
Yes. Some final days of December can belong to ISO week 1 of the next year.
Can a year have 53 weeks?
Yes. Some ISO week-numbering years contain a 53rd week.