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Week Number Calculator
Find the ISO week number and week-year for any date, including the Monday start and Sunday end of that week.
Find an ISO week number
See the ISO week and the Monday-to-Sunday week range.
Week number updates automatically when you change the date.
What is an ISO week?
ISO 8601 numbers weeks from Monday through Sunday. Week 1 is the week containing the year's first Thursday. Because of that rule, the first few days of January can belong to the previous ISO week-year, and the final days of December can belong to the next one.
DateTimeWise shows both the week number and week-year so that boundary cases are clear.