Date lookup
Week Number Calculator
Enter a date to find its ISO week number, ISO week-year, US week number, weekday number, and day of year.
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Choose date
Pick a date to calculate ISO week number, US week number, and day of year.
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Frequently asked questions
What is an ISO week number?
ISO week numbers use Monday as the first day of the week. Week 1 is the week containing the first Thursday of the ISO week-year.
Why can the ISO week year differ from the calendar year?
Dates near January 1 can belong to the last ISO week of the previous ISO week-year, or dates near December 31 can belong to week 1 of the next ISO week-year.
What is the US week number on this page?
The US week number uses Sunday as the start of the week and counts week 1 from January 1.