ISO Week Number Calculator

Calculate the ISO week number for any date. The ISO week date system is used mainly in government and business for fiscal years and planning.

Calculate Week Number

Select a Date:

Or Select Year and Week:

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About ISO Week Dates

ISO weeks start on Monday
The first week of the year is the week that contains the first Thursday
This means it's the week with January 4th or the week with the year's first Thursday
A year has either 52 or 53 ISO weeks
ISO week dates are represented as: YYYY-Www-D