There are four primary week numbering systems in use worldwide. Each system has subtle differences that you should be aware of. Excel can work with any of these systems.

ISO Week Numbers

Note: There is no built-in worksheet function for ISO weeks in Excel before Excel 2010, the formula and the function below will work in all Excel versions so it is a good option to use in your workbook if it will be used in different Excel versions. But FYI In Excel 2010(and also in Excel 2011 for the Mac) they add an argument of 21 to the Excel WeekNum function to get the ISO week number as result : =WEEKNUM(B4,21) and in Excel 2013 they finally add a new function named ISOWEEKNUM. So in Win Excel 2013 and higher and in Mac Excel 2016 and higher you can use the ISOWEEKNUM function.

Copy the worksheet function below and paste it in a worksheet cell and it check the Date in B4.

=INT((B4-DATE(YEAR(B4-WEEKDAY(B4-1)+4),1,3)+WEEKDAY(DATE(YEAR(B4-WEEKDAY(B4-1)+4),1,3))+5)/7)

You can also copy this UDF in a standard module and use this function =IsoWeekNumber(B4)

The Excel worksheet function WEEKNUM

Reliance on Analysis ToolPak is a major annoyance because the Add-in may not be installed or checked in Excel versions before Excel 2007. A default Excel installation has it unchecked. Also, there are international difficulties where you use ATP formulas because these formulas are not translated by Excel if you open the workbook in a different Excel language version.

Note: In Excel 2007 and up WEEKNUM is a Standard worksheet function so you not have the problems above if you share your workbook between different Excel 2007-higher language versions and if the add-in is unchecked.

You can use this two replacement functions from Daniel M to avoid problems above so they work in every Excel version.

Replacing =WEEKNUM(B4,1)
=1+INT((B4-(DATE(YEAR(B4),1,2)-WEEKDAY(DATE(YEAR(B4),1,1))))/7)

Replacing =WEEKNUM(B4,2)
=1+INT((B4-(DATE(YEAR(B4),1,2)-WEEKDAY(DATE(YEAR(B4),1,0))))/7)


Simple Week Numbering

Note: There is no built-in worksheet function for Simple week numbering in Excel.
Copy the function below and paste it in a worksheet cell.
=INT((B4-DATE(YEAR(B4),1,1))/7)+1

The week calendar file shows you all the dates and week numbers from a certain year on one printable page. If you want to have a week calendar from an other year you only have to change one cell (the year). There is a separate sheet for the following week numbering systems :

Information about Week Numbers Calendar
The Calendar is working with the 1900 and 1904 date system and don't have a problem with the Excel 1900 Leap Year error. Excel (for compatibility reasons) followed the error made by earlier spreadsheet packages which was to show a 29-Feb in 1900. Apart from now skipping 29-Feb-1900, our calendar also allows reading of the correct day names for dates before 1-Mar-1900. Although week numbers are unlikely to be wanted for 1900, it does now mean that there is now a source for a correct 1900 calendar.

We would like to acknowledge general reference on all date issues to: Chip Pearson, Dave McRitchie, Daniel Maher, John Green and the late Frank Kabel.