Free Week Number Calculator
Calculate week numbers from dates or find dates from week numbers. Supports both North American (Sunday start) and ISO 8601 (Monday start) week numbering systems with interactive calendar view.
How to Use the Week Number Calculator
- 1
Choose Calculation Mode
Select 'Date to Week' to find the week number for a specific date, or 'Week to Date' to find the date range for a specific week number.
- 2
Select Week Numbering System
Choose between North American system (weeks start Sunday, first week contains Jan 1) or ISO 8601 system (weeks start Monday, first week has at least 4 days).
- 3
Enter Your Input
For date-to-week mode, pick a date using the date picker. For week-to-date mode, enter the year and week number you want to look up.
- 4
View Results
See the calculated week number or date range instantly. The interactive calendar shows the week highlighted in context with surrounding dates.
Key Features
Dual Conversion Modes
Convert dates to week numbers or find specific dates from week numbers. Switch between modes with a simple toggle for maximum flexibility.
Multiple Week Systems
Support for both North American (Sunday start, Jan 1st week) and ISO 8601 (Monday start, 4-day rule) week numbering systems.
Interactive Calendar View
Visual calendar display showing the calculated week highlighted in context, making it easy to understand the week boundaries and surrounding dates.
Real-time Results
Automatic calculation as you type or change inputs. No need to click calculate buttons - results update instantly as you modify dates or week numbers.
What is a Week Number Calculator?
A week number calculator is a tool that converts between calendar dates and their corresponding week numbers within a year. Week numbering systems vary globally - the North American system starts weeks on Sunday with the first week containing January 1st, while the ISO 8601 international standard starts weeks on Monday with the first week containing at least 4 days of the new year. Our calculator supports both systems and provides an interactive calendar view to visualize the results.
Why Use Our Week Number Calculator?
- 1
Dual System Support
Works with both North American and ISO 8601 week numbering systems, making it useful for international business, project planning, and scheduling across different regions.
- 2
Bidirectional Conversion
Convert dates to week numbers or week numbers to dates with equal ease. Perfect for project management, scheduling, and calendar planning tasks.
- 3
Visual Calendar Context
Interactive calendar display shows your calculated week in context, helping you understand the week boundaries and plan around adjacent dates.
- 4
Instant Results
Real-time calculation means results update automatically as you change inputs. No need to click any calculate buttons - just immediate, accurate results.
Frequently Asked Questions
What's the difference between North American and ISO 8601 week systems?
North American weeks start on Sunday and the first week is the one containing January 1st. ISO 8601 weeks start on Monday and the first week must contain at least 4 days of the new year (meaning it contains January 4th).
Can I convert both ways - date to week and week to date?
Yes! Use the toggle buttons to switch between 'Date to Week' mode (find week number for a date) and 'Week to Date' mode (find date range for a week number).
Why do I get different week numbers for the same date in different systems?
The two systems have different rules for when weeks start and which week is considered 'week 1'. This can result in the same date having different week numbers, especially near the beginning and end of the year.
What does the calendar view show?
The interactive calendar displays the month containing your calculated week, with the specific week highlighted in color. It shows week numbers on the left and helps you visualize the week boundaries.
Are the results calculated in real-time?
Yes, the calculator updates results automatically as you change the date, year, week number, or numbering system. No need to click any calculate buttons - just modify your inputs and see instant results.