Calculate Week Number & Convert Dates
Find the current week number, convert dates, and determine date ranges for any week.
Today is Week
Convert Date to Week Number
Find Date Range for a Week
How Week Number Calculation Works
Premise: Determining the week number for a given date requires cross-referencing local calendar systems, specifically addressing the day the week begins (Monday vs. Sunday) and the definition of the first week of the year.
Evidence: By utilizing pure client-side JavaScript, our calculator evaluates the timestamp against both standard paradigms: The ISO 8601 standard (where Week 1 contains the first Thursday) and the US Standard (where Week 1 contains January 1st). The algorithm calculates the absolute millisecond delta between the target date and the start of Week 1, dividing by the constant duration of a week (604,800,000 milliseconds) to return the exact integer week.
Conclusion: This local algorithmic processing ensures that calculations are performed instantaneously in the browser without server latency, providing mathematically sound date-to-week conversions for project management and logistics.
How to Use the Week Number Calculator
- Find Today's Week: The current week number is automatically displayed at the top of the page when you load the tool.
- Convert a Date: Select any specific date in the calendar input to find out which week number it falls into, along with the start and end dates of that week.
- Find Dates by Week Number: Need to know when "Week 42" starts? Enter the year and the week number to instantly generate the date range.
- Change Standards: Toggle between the ISO 8601 standard (weeks start on Monday) and the US Standard (weeks start on Sunday) depending on your locale.
Frequently Asked Questions
What is the ISO 8601 Week Number Standard?
The ISO 8601 standard is the international standard for date and time formatting. In this system, weeks always start on Monday. The first week of the year (Week 1) is defined as the week containing the first Thursday of the year. This system is heavily used in European countries and global businesses.
How does the US Week Numbering differ?
In the United States, Canada, and several other countries, the calendar week traditionally starts on a Sunday rather than Monday. Furthermore, Week 1 is simply the week that contains January 1st, regardless of what day of the week it falls on.
How many weeks are in a year?
A standard year has 365 days, which roughly equals 52 weeks (52 x 7 = 364 days). A leap year has 366 days. However, depending on the day of the week the year starts and ends on, certain years contain a "Week 53" in both the ISO and US numbering systems.
Does this tool send my data to a server?
No, this Week Number Calculator is 100% client-side. All calculations are performed directly in your browser using JavaScript, meaning no data is ever sent to or stored on our servers.