Week Number Calculator
Find the ISO week number for any date.
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ISO Week Number
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Week Starts (Mon)
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Week Ends (Sun)
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Day of Year
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Weeks Left in Year
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Frequently Asked Questions
What is an ISO week number? +
ISO 8601 defines week numbering where Week 1 is the week containing the first Thursday of January. Weeks start on Monday and end on Sunday. A year has either 52 or 53 weeks.
Why does the ISO week year sometimes differ from the calendar year? +
Dates near the start or end of a calendar year can belong to a different ISO week year. For example, December 31 might be in Week 1 of the next year, and January 1 might be in Week 52 or 53 of the previous year.
How many weeks are in a year? +
Most years have 52 ISO weeks. Years where January 1 is a Thursday (or Thursday/Friday in leap years) have 53 weeks. This happens roughly every 5 to 6 years.
Do all countries use ISO week numbering? +
ISO week numbering is used in most of Europe and many international standards. The US and some other countries use a Sunday-start system where Week 1 contains January 1.
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