Calendar Week Calculator

The ISO 8601 standard defines a consistent way to number weeks throughout the year, starting on Monday and counting the week that contains the first Thursday as week 1. This system is widely used in Europe, North America, and industries such as manufacturing, logistics, supply chain management, and project planning. A calendar week calculator helps you quickly determine the ISO week number for any date and understand where it falls within the year's week structure.

Using this tool is straightforward: simply enter a date, and the calculator instantly reveals its ISO week number, the year associated with that week, and other useful week-related information. This is particularly valuable when coordinating schedules across regions, working with ISO-formatted reports, or syncing calendars with systems that use ISO week numbering. You can explore any date in history or the future to see how weeks align across years.

ISO weeks sometimes cross calendar year boundaries—for example, late December might belong to week 1 of the following year, and early January might belong to the previous year's final week. This tool handles those edge cases automatically, ensuring you always get the correct ISO week assignment. Whether you're a developer integrating ISO week logic, a project manager tracking schedules, or simply curious about how weeks work globally, this calculator provides instant, accurate results computed entirely in your browser.