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Week Number Calculator
Find the ISO week number for any date or get the date range for any week.
Date to Week Number
Week Number to Date Range
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Frequently Asked Questions
What is ISO week numbering?
ISO 8601 defines a week-based calendar where weeks start on Monday. Week 1 is the week containing the first Thursday of the year. This means week 1 always contains January 4th.
How many weeks are in a year?
Most years have 52 weeks, but some years have 53 weeks. A year has 53 weeks if it starts on a Thursday, or if it's a leap year that starts on a Wednesday.