Free Online Week Number Calculator Tool
Find the ISO week number for any date, or discover what dates fall within a specific week number. Perfect for project planning, payroll scheduling, manufacturing weeks, and academic calendars. 100% client-side.
ISO 8601 week date system · 100% client-side
How to Use the Week Number Calculator Tool
Pick a Date
Select a date to instantly see its ISO week number, year, and the Monday-to-Sunday range of that week.
Or Enter a Week Number
Switch to Week mode, enter a week number and year, and see the exact start and end dates for that week.
Navigate Freely
Use the previous/next buttons to move between weeks, or jump to the current week.
Powerful Week Number Calculator Features
Everything you need to calculate ISO week numbers — all in one free tool.
Date to Week Number
Enter any date and instantly see its ISO 8601 week number, week year, and day of the week.
Week to Date Range
Enter a week number and year to see the exact Monday-to-Sunday date range for that week.
ISO 8601 Standard
Uses the official ISO 8601 week numbering system where weeks start on Monday and week 1 contains January 4th.
Week Navigation
Navigate between consecutive weeks with previous/next buttons.
Current Week Shortcut
One-click jump to the current week number and dates.
Multiple Years
Supports any year. See week numbers across year boundaries correctly.
Copy Results
Copy the week number or date range to your clipboard.
100% Client-Side
All calculations happen in your browser. No server uploads.
Why Use Our Week Number Calculator Tool
The most accurate way to find ISO week numbers. Fast, reliable, and private.
100% Free
No hidden fees, no usage limits. Look up as many week numbers as you need.
ISO Standard
Follows the official ISO 8601 week date system used internationally.
Bidirectional
Convert dates to week numbers AND week numbers back to date ranges.
No Installation
Works entirely in your browser. No downloads, no plugins, no setup.
Key Highlights
- No sign-up required
- 100% client-side
- ISO 8601 compliant
- Date to week & week to dates
- Week-by-week navigation
- Works on any device
Frequently asked questions
What is an ISO week number?
The ISO week date system (ISO 8601) assigns a week number to each week of the year. Weeks start on Monday, and week 1 is the week containing January 4th. This system is widely used in business, manufacturing, and European date notation.
Is this week number calculator really free?
Yes, completely free. No hidden charges, no usage limits.
What's the difference between week number and week year?
The week year can differ from the calendar year. For example, January 1st 2023 falls in week 52 of 2022. The week year shows which year the week belongs to, not necessarily the calendar year of the date.
When does week 1 start?
According to ISO 8601, week 1 is the week that contains January 4th. This means week 1 always has the majority of its days (4 or more) in the new year. The first Thursday of the year also falls in week 1.
What are week numbers used for?
Week numbers are commonly used in project management, payroll cycles, manufacturing schedules, school timetables, and any context where weekly planning across year boundaries is needed.
Do I need to install anything?
No installation required. Works entirely in your web browser.
Can I use this on my phone?
Yes, fully responsive and works on all devices.