Week Number Calculator
Find the ISO 8601 week number (1-53) for any date, along with that week’s Monday-to-Sunday date range.
About the Week Number Calculator
The ISO 8601 standard defines week numbers that start on Monday, with week 1 being the week containing the year's first Thursday (equivalently, the week containing January 4th). This avoids the ambiguity of "week 1" meaning different things in different countries.
How it works: The calculator finds the Thursday of the input date's week, since that Thursday always falls in the ISO year the week belongs to, then counts how many full weeks have passed since that year's January 1.
ISO week numbers are widely used in business reporting, payroll, manufacturing schedules and project planning, since "week 33" is unambiguous in a way that "the week of August 10" is not. The calculator also shows the Monday-to-Sunday date range for that week.