Week number calculator
Find a date's ISO week number and week-year.
A date's calendar year can differ from its ISO week-year around New Year. This tool uses ISO 8601 week numbering, not Sunday-start calendars or organization-specific fiscal weeks. Change the input or settings to update your result. Results update as you type. Up to 1,000,000 characters.
Your result
How to use Week number calculator
- Enter the values you want to calculate and choose the relevant units or settings.
- The result updates as you type. Adjust the available settings if needed.
- Review the result, then use Copy result or a download link when available.
Useful next steps
How to use it
Choose a calendar date. ISO weeks start on Monday, and the first week of the year contains January 4. The result also shows the ISO weekday, Monday 1 through Sunday 7.
A worked example
January 1, 2021 belongs to ISO week 53 of 2020. January 4, 2021 starts ISO week 1 of 2021.
What to know
A date's calendar year can differ from its ISO week-year around New Year. This tool uses ISO 8601 week numbering, not Sunday-start calendars or organization-specific fiscal weeks.