Week Number Calculator
Get the ISO week number and day of year for any date.
Frequently Asked Questions
What is the ISO week numbering system?
ISO 8601 weeks start on Monday, and week 1 is the week containing the first Thursday of the year. This can put the last days of December in week 1 of the next year, or the first days of January in week 52 or 53 of the previous year.
How many weeks are in a year?
Most years have 52 ISO weeks. A year has 53 weeks when it starts on a Thursday, or starts on a Wednesday in a leap year. Businesses in logistics and manufacturing plan heavily around these week numbers.
Why does the US use a different week system?
US calendars often start weeks on Sunday and count week 1 from 1 January regardless of weekday. If you work with European partners or ISO standards, expect the ISO numbers this tool provides.
Is the Week Number Calculator free, and is my data private?
Yes. It is completely free and every calculation runs locally in your browser, so the numbers and dates you enter are never uploaded, stored, or shared with anyone.
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