Week Number Calculator
Find the ISO 8601 week number for any date.
What is an ISO week number?
ISO 8601 defines week 1 of a year as the week containing that year's first Thursday — equivalently, the week containing January 4. Weeks run Monday to Sunday. This means the first few days of January can sometimes belong to week 52 or 53 of the previous year, and the last days of December can belong to week 1 of the next year.
Where you'll actually see week numbers
ISO week numbers show up more often than most people realize: manufacturing date codes stamped on packaging, European payroll and invoicing cycles, sprint planning in software teams, and academic or fiscal calendars that run "week 1 through week 52" instead of by month. They're rarely used in everyday US context, which is usually why someone lands here trying to decode a "week 23" reference from a supplier or spreadsheet.
Frequently asked questions
What is the ISO 8601 week numbering system?
It's the international standard that defines a week as running Monday to Sunday, with week 1 being whichever week contains the first Thursday of the year.
Why does week 1 sometimes start in late December?
Because week 1 is defined by containing the year's first Thursday, not by containing January 1 — so if January 1 falls on a Friday, Saturday, or Sunday, the days before it can belong to week 1.
How many weeks are in a year?
Most years have 52 weeks, but years where January 1 falls on a Thursday (or a leap year where it falls on a Wednesday) have 53 ISO weeks.