Equable means unvarying, free from extremes, or not easily disturbed. Equatable (pronounced ee-QUATE-uhble) means capable of being equated (usually applied to two or more comparable things). Equitable (pronounced EH-quit-uhble) means fair, impartial, or proportionate.
Both equable and equatable are closely related to equal, though equable is derived not from equal but from the words’ shared root—the Latin aequus, meaning even or level.1 2 Equitable is distantly related to these words, but it comes directly from equity, which means (among other things) the state or quality of being just and fair.1
Your spell check might try to make you change equatable to equitable, but spell check is wrong on this one. Equatable is a perfectly good word. It can cause confusion, however, because it’s much rarer than equitable. Unless context makes equatable‘s meaning clear, consider synonyms such as similar, analogous, or comparable.
Equable means unvarying or free from extremes—for example:
Throughout world history, tyrants and despots vastly outnumber equable and magnanimous rulers of temperance. [GhanaWeb]
Days is more equable and consistent, hanging onto a rhythmic ostinato while rising to a hefty climax before sinking back into silence. [Guardian]
His assets are that equable temperament that will allow him to remain calm when the flak starts flying. [Scotsman]
These writers correctly use equitable as a synonym of fair, impartial, or proportionate:
[T]he only way to build an equitable society is to make sure everyone has an equal say. [New York Magazine]
[I]t is time for all political parties to commit to an equitable approach to education funding. [Toronto Star]
[I]t must be set at a level which rewards the players with an equitable share of the revenue they help generate. [Sydney Morning Herald]
And here are two examples of the comparatively rare equatable put to use:
I don’t recommend going there, as it’s equatable to the Mos Eisley cantina of the internet. [Crave Online]
Burr said he had no idea whether homosexuality is a choice or biological and bristled at the idea that the battle for racial Civil Rights is equatable to granting LGBT rights. [Talking Points Memo]