Week Number Calculator

ISO Week Number

ISO week numbers start on Monday, and week 1 is the week containing the first Thursday of the year.

Method: adjust the date to its Thursday, then count weeks from the first Thursday week.

Understanding Week Numbers

1. What This Calculator Measures

ISO week numbers label weeks in a year using Monday as the first day of the week.

2. How the Calculation Works

The calculator adjusts the selected date to its Thursday and counts ISO weeks from the start of the ISO year.

3. Formula or Rule Used

ISO week 1 is the week containing the first Thursday.

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