What is the plural of radius?

Hello, Eliana! 
Such a good question, with an even more interesting answer. 

'Radius' is a word incorporated into English from Latin word radius meaning the spoke of a chariot wheel.
Therefore, it does not follow the general rules of adding '-s' or '-es' to form its plural. It simply replaces “es” with 'i'.
The plural of radius can be either 'radii' (from the Latin plural) or the conventional English plural 'radiuses'.
The plural form more common for conventional English speakers is 'radiuses'.

Similar words that are made plural this way are abacus (abaci), alumnus (alumni) and cactus (cacti).