ISO Week Number Calculator
Find a date’s ISO week number, ISO week-year, weekday and the Monday–Sunday boundaries of its week.
Set your values
Adjust the values, then calculate your result.
How to use ISO Week Number Calculator
- Load the example or enter your own values.
- Choose the relevant options and run the tool.
- Review the result, then copy it or download a local file.
When this tool is useful
Week numbers in reports can differ from the calendar year near New Year. This calculator returns the ISO week date, where Monday is day one and week one contains January fourth. Use it to label a weekly export, verify an analytics grouping or identify the Monday and Sunday belonging to a particular date.
Reading the result and limitations
The ISO week-year is shown separately from the ordinary calendar year because late December can belong to the next ISO year and early January can belong to the previous one. Output dates use the Gregorian calendar and are calculated in UTC without relying on your device time zone. This tool does not implement country-specific Sunday-start week numbering or fiscal calendars. Supply a complete valid date rather than a year and week alone.
What are the main limits of ISO Week Number Calculator?
The ISO week-year is shown separately from the ordinary calendar year because late December can belong to the next ISO year and early January can belong to the previous one. Output dates use the Gregorian calendar and are calculated in UTC without relying on your device time zone. This tool does not implement country-specific Sunday-start week numbering or fiscal calendars. Supply a complete valid date rather than a year and week alone.
Is input uploaded?
Processing takes place in your browser. This tool does not send your input or selected files to a conversion API.