ISO week number calculator
Many businesses, schools and factories plan by week number instead of by date. This tool gives the ISO 8601 week number for any date, along with the week-year and weekday. It is handy for project schedules, payroll calendars and reports labeled like 2024-W10.
- Runs in your browser
- Free, no account needed
- Copy or download the result
Result
Example
2021-01-01
ISO week: 2020-W53 ISO weekday: 5 (Monday = 1, Sunday = 7)
How to use the iso week number calculator
- Type a date as YYYY-MM-DD, such as 2021-01-01.
- Press Run to look up the week.
- Read the ISO week, written as year-W and two digits, and the weekday number.
- Copy result or Download it for your records.
How it works
ISO weeks start on Monday. Week 1 is the week that contains January 4.
Dates near New Year can belong to the previous or next week-year. For example, 2021-01-01 is in 2020-W53.
The weekday is numbered with Monday as 1 and Sunday as 7.
This uses the ISO rule only. It does not show US-style weeks that start on Sunday.
Everything runs in your browser. Your input is not uploaded or stored.
Questions and answers
How are ISO week numbers calculated?
Each week runs Monday to Sunday, and week 1 is the one that holds the year's first Thursday, which is also the week with January 4. A year has 52 or 53 ISO weeks.
Why does January 1 sometimes show week 52 or 53?
If January 1 falls on a Friday, Saturday or Sunday, that day still belongs to the last week of the previous ISO year.
What does 2024-W10 mean?
It means ISO week 10 of the 2024 week-year. The week-year can differ from the calendar year for a few days around New Year.
Does this tool count weeks starting on Sunday?
No. It follows ISO 8601, where weeks always start on Monday.