See a date’s week number, week-year, weekday position, and start and end dates.
Calculate with the Week Number Calculator
Dates are validated as calendar components before calculation.
Week Number Calculator results
How to use the Week Number Calculator
- Enter the required calendar dates, times, or rule values.
- Choose the counting convention or mode where offered.
- Select Calculate or press Enter.
- Review the result, basis, and limitations before using it.
Worked example
31 December can belong to ISO week 1 of the following week-year.
Assumptions
Date-only calculations use the proleptic Gregorian calendar and ignore clock time. Elapsed-time calculations and selected-zone conversions state their time basis separately.
Limitations
- Non-ISO systems vary across software and locales.
- This page documents the implemented week-1 convention.
Frequently asked questions
What date basis does the Week Number Calculator use?
The result uses calendar dates unless the page explicitly describes elapsed time or a selected time zone.
Can daylight-saving time change calendar-day totals?
No. Date-only calculations use normalized UTC day numbers. Elapsed timestamp and time-zone calculations are explicitly handled separately.
Are entered dates stored?
No. Calculations run in your browser and the entered dates are not saved or sent anywhere.