There are 2 functions to calculate the week number according to a date in Excel
- The WEEKNUM function
This is the original function that calculated the week number in Excel. But it doesn't respect the conventional rules of most countries in the world.
- The ISOWEEKNUM function
To avoid calculation errors, Microsoft has created another function to calculate the week number.
The WEEKNUM function
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 the 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 week number according to the American rule.
That's good for the people who work in the USA but NOT for the other countries
The difference between the 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 the USA it will be 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 uses the ISO 8601 rule to calculate the week number, and the USA uses its own rule.
The ISOWEEKNUM Function
To 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 results for January the 1st for these different years when using WEEKNUM or ISOWEEKNUM.
And the difference is more obvious with the date of the 4th January.
As you can see, in 2021 and 2022, both formulas don't return the same week number for the 4th of January. That means there will be a difference in your dashboard if you don't use the correct function according to the convention used in your country.
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
In this version, you can add a second parameter to the function WEEKNUM.
If you put in the value 21 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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