What is an ISO week number?
The ISO week date system numbers weeks from Monday through Sunday. The first week of an ISO year is the week containing the first Thursday of that year, so an ISO week-year can differ from the calendar year near New Year.
Formula and method
ISO weeks start on Monday; week 1 contains the first Thursday of the year. This calculator shifts the chosen date to its week's Thursday, identifies the ISO week-year, then determines the week number within that year.
How to use this calculator
- Choose a calendar date.
- Select Calculate.
- Read the ISO week number and ISO week-year.
Accuracy and limitations
The result follows ISO week numbering. It reports the week-year as well as the week number because dates near the start or end of a calendar year may belong to a different ISO week-year.