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 specialized function to determine the ISO week number. This example provides the ISO week number for whatever date is in cell A1:

=ISOWEEKNUM(A1)

The ISOWEEKNUM function was introduced in Excel 2013, so if you are using an earlier version of Excel (or your workbook will be used by others with earlier versions of Excel), then you cannot rely on it. Instead you'll 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, you'll find this page helpful:

http://www.cpearson.com/Excel/WeekNumbers.aspx

ExcelTips is your source for cost-effective Microsoft Excel training. This tip (7847) applies to Microsoft Excel 2007, 2010, 2013, 2016, 2019, Excel in Microsoft 365, and 2021. You can find a version of this tip for the older menu interface of Excel here: ISO Week Numbers in Excel.