When to use this tool
- Label weekly reports.
- Align sprint and operations calendars.
- Find the day of year for logs or reports.
Calendar
Week number calculator
Pick a date and see its ISO week, ISO week-year, day of year, and weekday.
ISO week
26
ISO week-year
2026
Day of year
178
Weekday
Saturday
ISO Week Number Calculator: definition and examples
An ISO week number identifies a week using the ISO 8601 week-date calendar.
How to use it
- Choose a date.
- Read the ISO week and ISO week-year.
- Copy the day-of-year or weekday value.
Example
ISO week 1 is the week containing the first Thursday of the calendar year.
Frequently asked questions
Can the ISO week-year differ from the calendar year?
Yes. Dates near New Year can belong to the previous or next ISO week-year.
Which day starts an ISO week?
ISO weeks start on Monday.