Week Number Calculator

The Week Number Calculator identifies the ISO week number of any date and displays the full date range of that week. ISO 8601 week numbering is the international standard used in business reporting, project scheduling, accounting periods, manufacturing planning and logistics. Week 1 of the ISO year is the week containing the first Thursday of January, which means ISO years can start in late December of the previous calendar year or early January.

ISO weeks always begin on Monday and end on Sunday. The ISO year can have 52 or 53 weeks — years with 53 ISO weeks occur when January 1 falls on Thursday (or on Wednesday in a leap year). This means that December 29, 30 and 31 can belong to ISO Week 1 of the following year, and January 1, 2 or 3 can belong to ISO Week 52 or 53 of the previous year. This system differs from US and ISO calendar conventions where weeks begin on Sunday, causing different week numbers for the same dates.

For project planners and business analysts, ISO week numbers provide a consistent reference frame for scheduling deliverables, billing cycles, reporting periods and capacity planning. Many European countries and multinational corporations use ISO weeks as the primary date reference in internal documents. Use our Date Add/Subtract Calculator to jump a specific number of weeks forward or back, and our Days Between Dates to count the weeks between two dates. Free, no registration required.