What is the plural of Genius? Geniuses or Genii?

Geniuses is the plural of the adjective genius. Genii is no longer common and should not be used in English.

The class is full of geniuses. 


When we have a word that ends in “s” and we want to make it plural, we use -es. 

There are not that many geniuses in the world.

Other plural words that end in -es include buses and misses.  


Some people use “genii” as the plural for “genius” because there are certain words that come from Latin that use “i” for the plural form like cactus(cacti is the plural form)

Genius’s or Genius’

If you need to use “genius” as a possessive then you can use either genius’s or genius’. It just depends on the style guide that you follow. 

The book is called “ The Genius’s Guide to English”.

The book is called “ The Genius’ Guide to English”.