Week number
Pick a date to see its ISO week number, or a week number to see its dates.
Frequently asked questions
How are ISO week numbers counted?
Weeks start on Monday, and week 1 is the week with the year’s first Thursday (the week that contains 4 January). So 1–3 January can belong to the last week of the previous year.
Does every year have 52 weeks?
Most do; years that start on a Thursday, and leap years that start on a Wednesday, have 53.
Is the US week number the same?
Not always. US calendars often start weeks on Sunday and count the week of 1 January as week 1, which can differ by one from ISO.