Usable is the antonym of the word unusable. It refers to something that is fit or appropriate for use. Also, functional is an antonym that suggests something works effectively, as opposed to being unusable. Another antonym of unusable is serviceable, which means being capable of being of assistance or use. Functional, practical, and accessible are some other antonyms that can be used for unusable. Suitable, useful, and operable would also work as antonyms for unusable. If something is in good condition and can be used as intended, it is considered the antithesis of unusable.