Method
How the week number calculator works
ISO-8601 numbers weeks from Monday to Sunday and assigns week 1 to the week containing the first Thursday of the year. The calculator anchors on that Thursday rather than on 1 January.
Counting clock time within a day rather than calendar days? The hours calculator measures the hours and minutes between two times, the time card calculator totals a week of shifts, the add time calculator adds or subtracts durations, and the military time converter switches between 24-hour and AM/PM times. The rest are in the time calculators collection.
Example
Week number calculator example
Early-January dates can fall in week 52 or 53 of the previous ISO week-numbering year, which is why the result states the week-numbering year explicitly.
FAQ
Week number calculator FAQ
Why does the week-numbering year sometimes differ from the calendar year?
Under ISO-8601 the first days of January can belong to the final week of the previous year, and late December can belong to week 1 of the next.
Do all countries number weeks this way?
No. Some business and broadcast calendars start weeks on Sunday or Saturday, which shifts the numbering. This page uses ISO-8601.
How many weeks can a year have?
An ISO year has 52 or 53 weeks depending on which weekday the year starts on and whether it is a leap year.