Incorrect spelling

convertable

Incorrect spelling, explanation: suffix -able is generally used with words that, without a suffix, are in their full form (just like understandable without the suffix is understand – we don’t change anything). Convert is such word, but it is an exception. Since its Latin-origin word we use suffix -ible with it, so convertible is the correct form.


Correct spelling

convertible

Correct spelling, explanation: usually with Latin-origin words we use suffix -ible when creating an adjective. Convert is one of such words, so convertible is the correct spelling. It is sometimes said that suffix -able is added to words that are left full without the suffix (such as dependdependable), and this is generally true, so that may be why the confusion with convertable exists, however this rule has exceptions, and convertible is one of them.

Definition of convertible:
adjective, something adaptable, able to be changed
Look how convenient this convertible bag is – it can be a handbag or a backpack!
noun, a type of car with roof that folds
Joe was planning to buy a coupé, but finally he has bought a convertible.