The Correct Value for a Week Number

The function WEEKNUM

In Excel, to return the week number of a date you have the function WEEKNUM.


Easy ? Sure ? 🙄🤔 Well in fact, it's not so simple. It depends if you are in USA or in another country.

The rule of calculation for the week is different between the USA and the rest of the world.

And guess what, because the function has been created by Microsoft, an american company, the function returns the american rule.

That's good for the people who work in USA but for the other it's not correct

Rule to calculate the week number in the world

The difference between USA and the rest of the world is simple.

  • In USA, whatever the number of days in the first week of the year, it's the week #1
  • In the rest of the world, the first week must have a least 3 days.

So, if the 1st January is a Saturday, in USA it will be the week#1 but in the rest of the world the week number will be the value of the last week number of the previous year (week #53)

To sum up, the rest of the world use the ISO 8601 rule to calculate the week number and the USA use their own rule.


So correct this and to be able to return the correct week number for all the countries in the world, Microsoft provided the function ISOWEEKNUM, in
version 2013 and later.


Look at the difference in result on the for January the 1st for these different years when using WEEKNUM or ISOWEEKNUM.

And for the 4th of January

How to calculate the ISO week number in Excel 2013 and previous versions

Excel 2007 and previous versions

The formula to calculate the ISO week number is very complex but it works =INT((TODAY()-SUM(MOD(DATE(YEAR(TODAY()-MOD(TODAY()-2;7)+3);1;2);{1E+99;7})*{1;-1})+5)/7)

Excel 2010

In this version, you can add a second parameter to the function WEEKNUM.

If you put in the value 21 (hmmm, ok, why not 🤔🙄) for the second parameter, the WEEKNUM function will return the same result as the function ISOWEEKNUM does in Excel 2013 and later versions.


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