Week Number Calculator
Find the ISO 8601 week number for any date.
Last updated
—ISO week number
—ISO week-year
—Weeks in that ISO year
—Day of week
About this tool
ISO 8601 weeks run Monday to Sunday, and week 1 of a year is defined as the week containing that year's first Thursday — equivalently, the week containing January 4th. Because of this, the first few days of January can fall in the final week (52 or 53) of the previous ISO year, and the last few days of December can fall in week 1 of the next one — which is why this tool shows an "ISO week-year" separately from the calendar year. Most years have 52 weeks; years where January 1st is a Thursday (or a leap year where it's a Wednesday) have 53.
Nothing entered here is sent to a server — the entire calculation runs as JavaScript in your browser tab.