Please Note: This article is written for users of the following Microsoft Excel versions: 97, 2000, 2002, and 2003. If you are using a later version (Excel 2007 or later), this tip may not work for you. For a version of this tip written specifically for later versions of Excel, click here: ISO Week Numbers in Excel.
Excel provides a function that can be used to calculate the week number, of the year, of any given date. How you can use the WEEKNUM function has been recounted in other ExcelTips. There are some methods of determining weeks where the WEEKNUM function won't do, however. For instance, you may need to find the week number based on the ISO method of calculating weeks.
In the ISO scheme of things, weeks are calculated based upon the start of the week being Monday. Not so tough, right? However, the first week of the year is defined as beginning on the Monday of the week in which the first Thursday of January occurs. Whoa! This means that the first week of the year can actually begin on Monday, December 29, of the preceding year and it is possible that the last week of the preceding year can end as late as Sunday, January 3, of the current year.
This is too much for WEEKNUM to handle. Instead, you will need to use a formula to calculate the ISO week number for the year:
=INT((A1-DATE(YEAR(A1-WEEKDAY(A1-1)+4),1,3)+ WEEKDAY(DATE(YEAR(A1-WEEKDAY(A1-1)+4),1,3))+5)/7)
If you need more information on the how to handle ISO week numbers and dates in Excel, refer to these sources:
http://www.rondebruin.nl/win/s8/win002.htm http://www.cpearson.com/Excel/WeekNumbers.aspx
ExcelTips is your source for cost-effective Microsoft Excel training. This tip (7844) applies to Microsoft Excel 97, 2000, 2002, and 2003. You can find a version of this tip for the ribbon interface of Excel (Excel 2007 and later) here: ISO Week Numbers in Excel.
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Please Note:
This article is written for users of the following Microsoft Excel versions: 97, 2000, 2002, and 2003. If you are using a later version (Excel 2007 or later), this tip may not work for you. For a version of this tip written specifically for later versions of Excel, click here: 
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