Calculate week numbers from any date or find the date range for any week number. Supports both North American and ISO 8601 week numbering systems. Perfect for project planning, scheduling, and calendar management. No Signup Required.
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The choice of week numbering system can affect project deadlines, payroll periods, and business reporting. Always verify which system your organization uses before making important scheduling decisions.
The concept of numbering weeks within a year has ancient roots, but modern standardization came much later. The ISO 8601 standard was established in 1988 to create international consistency for date and time representation.
Before standardization, different countries and industries used various methods to count weeks, leading to confusion in international business and communication.
Today, week numbers are essential in manufacturing (production schedules), retail (seasonal planning), and software development (sprint cycles), making accurate calculation tools like this one crucial for modern business operations.
This calculator implements both week numbering algorithms with mathematical precision. The North American system uses modular arithmetic based on January 1st, while ISO 8601 uses the "Thursday rule" - week 1 contains the first Thursday of the year.
The calendar generation algorithm accounts for month boundaries, leap years, and proper week highlighting across different month views, ensuring accurate visual representation.
All calculations are performed client-side using JavaScript's Date object, with careful handling of timezone considerations and edge cases like December 29-31 potentially belonging to week 1 of the following year in ISO 8601.
North American week numbering starts weeks on Sunday and considers the first week of the year to be the one containing January 1st. ISO 8601 starts weeks on Monday and defines the first week as the one with at least 4 days in the new year, which means it always contains January 4th.
A year has 53 weeks when January 1st falls on a Thursday (in ISO 8601) or when January 1st falls on a Saturday or Sunday in a leap year (North American). This happens because 365 days don't divide evenly by 7, leaving 1 extra day, or 2 in leap years.
Use North American if you're in the US or Canada, or working with systems that follow Sunday-to-Saturday weeks. Use ISO 8601 for international business, European contexts, or when you need the Monday-to-Sunday standard used in most of the world.
This calculator is completely accurate for both North American and ISO 8601 week numbering systems. It correctly handles leap years, year boundaries, and all edge cases according to the official standards.
Absolutely! Week numbers are commonly used in project management, manufacturing schedules, academic calendars, and business planning. This tool helps you convert between specific dates and week numbers for better scheduling.
In North American system, week 1 contains January 1st. In ISO 8601, week 1 is the first week with at least 4 days in the new year, which means it always contains January 4th. This can sometimes mean week 1 actually starts in December of the previous year.
Simply enter the date in the 'Date → Week' mode, select your preferred week numbering system, and the calculator will instantly show you the week number, along with the full week's date range and a calendar view.