Week Number Calculator
Find the ISO week number, ISO week-year, US week number, day of year, quarter, and start/end dates for the ISO week.
Free local date utility. No sign-up and no server-side processing.
ISO Week
US Week Number
Day of Year
Quarter
ISO Week Starts
ISO Week Ends
ISO week-year edge cases
Dates near January 1 and December 31 can belong to the previous or next ISO week-year. This calculator shows the ISO week-year explicitly so reporting periods are easier to label.
FAQ
Do ISO weeks start on Sunday? No. ISO weeks start on Monday.
Why can January 1 be week 52 or 53? ISO week-years follow week boundaries, not calendar-year boundaries.