Find the ISO week number for any date — with week start and end dates, day of year, quarter, and year progress. Defaults to today so you always know what week it is right now.
An ISO week number is defined by the ISO 8601 standard. Weeks run Monday to Sunday, and Week 1 is the week containing the year's first Thursday — or equivalently, the week containing 4 January. This means the ISO week year and the calendar year don't always match at the very start and end of January.
Because ISO weeks start on Monday, the first few days of January can belong to the last week of the previous ISO year. If 1 January falls on a Friday, Saturday, or Sunday, it still belongs to the week that started in December — so it gets week 52 or 53 of the previous year.
Most years have 52 ISO weeks. Some years have 53 weeks — this happens when 1 January falls on a Thursday, or on a Wednesday in a leap year. Years with 53 weeks include 2015, 2020, 2026, and 2032.
Week numbers are widely used in Scandinavia for everyday scheduling, and across Europe in logistics, manufacturing, and project management. They appear on Gantt charts, fiscal calendars, delivery windows, and production schedules. Retail, finance, and engineering industries often reference weeks and quarters rather than specific dates for internal reporting.