About this ISO Week Number Calculator
ISO Week Number Calculator identifies the standardized Monday-based week number used in many business and technical systems.
Formula or method
ISO weeks start Monday; week 1 is the week containing the year’s first Thursday.
Example
January 4 always falls in ISO week 1.
Common uses
- Find an ISO week for planning.
- Check week-based reporting dates.
- Understand dates that belong to the previous or next ISO week-year.
How to use this tool
- Enter the values or text requested by the calculator.
- Choose units or options when the tool provides them.
- Select the calculation button and review the result.
- Use the formula, example and notes on this page to check whether the result fits your situation.
Important note
The ISO week-year can differ from the calendar year for dates near New Year.
Frequently asked questions
Can December dates belong to week 1 of the next year?
Yes. ISO week-years can cross calendar-year boundaries.
What day starts an ISO week?
Monday.