Find the ISO 8601 week number and day of the year for any date.
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This calculator uses the ISO 8601 standard, where weeks start on Monday and week 1 of the year is defined as the week containing the year's first Thursday. This means the first few days of January can sometimes belong to the last week of the previous year, and the last few days of December can sometimes belong to week 1 of the following year.
ISO week numbers are widely used in business, manufacturing and project planning for consistent weekly reporting, since every ISO week has exactly 7 days and starts on the same weekday (Monday), unlike calendar weeks which can be split unevenly across months.
Near the start or end of a calendar year, a date's ISO week can belong to the adjacent year's week numbering, for example, December 31 might fall in week 1 of the following year, or January 1 might fall in the last week of the previous year.
This calculator follows the ISO 8601 standard, where weeks start on Monday. This differs from some calendars and regions that start the week on Sunday.
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