ISO Week Numbers

How ISO weeks work

ISO 8601 weeks start on Monday, and week 1 is the week containing the year's first Thursday — equivalently, the week containing 4 January. That means up to three days of early January can belong to the previous year's final week (week 52 or 53), and a "week-year" can differ from the calendar year. Long years with 53 weeks occur roughly every 5–6 years.

Week numbers are the planning currency of much of Europe and of retail supply chains. In code, beware locale-dependent week functions — many default to Sunday-start US weeks. See language-specific epoch and date handling in our code snippets, or convert dates with the epoch converter.