ISO Week Calculator
Select any calendar date to find its corresponding ISO 8601 week number and ISO year.
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About ISO Week Standards
What is an ISO week?
The ISO 8601 standard defines weeks starting on Monday. Each week's year is the Gregorian year in which Thursday falls, meaning early January or late December dates can belong to adjacent ISO years.
How many weeks are in an ISO year?
Most years have 52 ISO weeks, but years that start on a Thursday (or leap years starting on a Wednesday) contain 53 ISO weeks.