Week Number Calculator
Free online week number calculator. Find the ISO week number for any date, plus US and Islamic week numbering systems.
About the Week Number Calculator
Find a date’s ISO week number or choose Gregorian week counting with Sunday or Saturday starts. ISO week one contains the first Thursday and can belong to a different year than the calendar date. The other systems count the week containing January 1 as week one. Saturday-start mode is Gregorian, not a Hijri calendar calculation.
Features
- ISO week and week-year
- Sunday-start Gregorian counting
- Saturday-start Gregorian counting
- Week date range
How to use this tool
The Week Number Calculator works right in your browser — no signup, no installation. Follow these steps:
- Select a date using the date picker or click Today.
- View the week number in all three numbering systems.
- See the week date range and total weeks in the year.
- Switch between systems to compare week numbering.
Frequently asked questions
How does this tool work?
Find a date’s ISO week number or choose Gregorian week counting with Sunday or Saturday starts. ISO week one contains the first Thursday and can belong to a different year than the calendar date. The other systems count the week containing January 1 as week one. Saturday-start mode is Gregorian, not a Hijri calendar calculation.
Using the results
The Week Number Calculator gives you exact dates and durations without manual counting. It calculates instantly in your browser, accounts for the cases people actually need, and keeps whatever you enter on your own device.