No servers
Inputs remain on your device.
Week Number Calculator
Find simple calendar week, ISO week and day of year for any date.
Calculations happen locally; dates and time zones are not transmitted.
How to use the week number calculator
- 1
Enter the required dates, times or options.
- 2
Select Calculate to run the operation locally.
- 3
Review and copy each format or print the result.
Calendar accuracy in your browser
Explicit rules
Validates ranges, leap years and month changes.
International formats
Uses Intl and IANA to localize results.
Frequently asked questions
Why can week numbers differ?
The simple calendar week and ISO week use different rules for the first week and week start.
Are my dates sent to a server?
No. Every operation runs in your browser.
Which calendar is used?
It uses the Gregorian calendar and dates from years 1 through 9999.