Week Number Calculator
Calculate week numbers instantly using ISO 8601 or North American systems. Convert between dates and week numbers, view interactive calendars, and understand both week numbering systems. Perfect for project planning, scheduling, and international coordination.
How to Use the Week Number Calculator
- 1
Choose Conversion Mode
Select 'Date to Week' to find the week number for a specific date, or 'Week to Date' to find which dates fall in a specific week.
- 2
Select Week Numbering System
Choose ISO 8601 (international, Monday-based) or North American (Sunday-based) depending on your regional preference or requirements.
- 3
Enter Your Information
For date to week: pick a date using the date picker. For week to date: enter the year and week number you want to convert.
- 4
View Results
Results appear automatically! You'll see the week number (for date mode) or the date range for that week (for week mode).
- 5
Check the Calendar
Use the interactive calendar below to visualize the week numbers and see your selected date/week highlighted.
Key Features
Dual Conversion Modes
Convert from date to week number or from week number back to dates. Both conversion directions work seamlessly with automatic validation.
Two Week Numbering Systems
Choose between ISO 8601 (international standard, weeks start Monday) or North American system (weeks start Sunday) for accurate regional calculations.
Interactive Calendar View
Visual calendar displays week numbers alongside dates, with the selected week/date highlighted for easy reference and understanding.
Instant Results
Results update automatically as you change dates, weeks, or systems. No need to click a calculate button - it's all real-time.
Week Date Range Display
When converting week to date, see the complete date range for that week (start and end dates) based on your selected system.
Current Date Support
Automatically loads the current date and its week number, making it easy to find out what week it is right now.
What are Week Numbers?
Week numbers are a standardized way to refer to specific weeks of the year, numbered from 1 to 52 (or 53 in some years). Different systems exist: ISO 8601 (used internationally) defines weeks as Monday-to-Sunday with week 1 being the first week containing a Thursday, while the North American system uses Sunday-to-Saturday weeks with week 1 starting on January 1st. Week numbers are commonly used in business, project management, manufacturing, and logistics for scheduling and planning purposes. They provide a universal reference that's more convenient than saying 'the week of January 15th.'
Why Use Our Week Number Calculator?
- 1
Dual System Support
Only tool you need for both ISO 8601 and North American week numbering systems, perfect for international collaboration.
- 2
Bidirectional Conversion
Convert in both directions - from dates to weeks and from weeks to dates - all in one convenient tool.
- 3
Real-Time Updates
Instant automatic calculations as you make changes. No waiting, no button clicking - just immediate results.
- 4
Visual Calendar
Interactive calendar view helps you understand week numbers visually and see the entire month context at once.
- 5
Business Planning
Perfect for project management, sprint planning, production scheduling, and any business that operates on weekly cycles.
- 6
Educational Tool
Learn the differences between week numbering systems and understand how week numbers work with clear visual examples.
Frequently Asked Questions
What's the difference between ISO 8601 and North American week numbering?
ISO 8601 (international standard) uses Monday as the first day of the week, and week 1 is the first week containing a Thursday (or January 4th). North American system uses Sunday as the first day, and week 1 simply starts on January 1st. This can result in different week numbers for the same date!
Why do some years have 53 weeks?
In the ISO 8601 system, years that start on Thursday or leap years that start on Wednesday have 53 weeks. This happens because week 1 must contain January 4th, and week 52 must contain December 28th - the math occasionally creates a 53rd week.
Which week numbering system should I use?
Use ISO 8601 for international contexts, European business, or technical standards. Use North American if you're in the US/Canada or working with systems that follow Sunday-based week numbering. The tool supports both!
What week number is it right now?
The calculator automatically loads the current date when you open it, showing you the current week number in both systems. Just select your preferred system to see today's week number.
How do I find what dates are in week 15?
Switch to 'Week to Date' mode, enter the year and week number (15), select your system (ISO 8601 or North American), and you'll see the complete date range for that week.
Can I use this for historical dates?
Yes! The calculator works for any year and week number. Enter any date or week from the past or future to get accurate week number calculations.
Why does the calendar show week numbers on the side?
The 'W' column shows the week number for each row of dates, making it easy to see which week number corresponds to which dates at a glance.
Do week numbers reset every year?
Yes, week numbering restarts at week 1 for every new year. However, note that in ISO 8601, the last few days of December might be in week 1 of the next year, and the first few days of January might be in week 52/53 of the previous year.