Week Number Calculator
Free online week number calculator. Find the ISO week number for any date, plus US and Islamic week numbering systems.
About This Free Online Week Number Calculator
Our free week number calculator tells you which week of the year any date falls in, using the ISO 8601 standard where weeks start on Monday and week one contains the year's first Thursday. Pick a date and the tool returns its week number instantly, which is essential for business planning, payroll and scheduling, project timelines, and anyone whose work refers to weeks rather than calendar dates. It also works in reverse so you can see which dates a given week number covers. Because it follows the ISO standard, the result matches the week numbering used across Europe and in most business software. The calculator runs entirely in your browser with instant results, so there is no signup, and nothing you enter is sent to a server.
Features of Our Week Number Calculator
- ISO 8601 week number for any date
- US/North American week numbering system
- Islamic week numbering system
- Week date range (start and end dates)
- Total weeks in selected year
- Real-time results as you pick a date
How to Use the Week Number Calculator
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 About Week Number Calculator
What is ISO week date?
ISO 8601 week numbering defines week 1 as the week containing the first Thursday of the year. Weeks start on Monday. This is the international standard used in most European and Asian countries.
What is the US week numbering system?
In the US system, week 1 is the week containing January 1. Weeks start on Sunday. This system is used in the United States, Canada, and most of Latin America.
How many weeks are in a year?
A year has either 52 or 53 weeks. There are 52 weeks plus 1 day (or 2 in a leap year), which means some years have a 53rd week depending on the numbering system.
Why Choose the Week Number Calculator?
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.