At the request of my husband's grandmother--Gram, as she is best known, I am writing about a confusing little piece of spelling, one of those that's hard to discern by sound cues. Lots of words end in -ible or -able, which sound almost identical in most American accents. I decided to look it up for myself, since spelling is oddly natural to me (except when I try to write without putting in my contacts!) and I've never had a problem differentiating, even though I have no idea what the pattern or rule is when it comes to using -ible and -able. So I ventured over to the online writing lab (OWL) at Purdue, which is one of the best resources you can possibly use for rules on writing and formatting, and discovered that the rule is astonishingly simple for any rule in the English language. This is it: if the root word is a complete word that can stand on its own, use -able. If not, use -ible. For good measure, I'll give you a few examples.
Take the word "manageable." You know you can write or say "manage" as a complete word, so "manageable" is correct.
However, "incredible" uses the -ible ending because "incred" is not a complete word; it must be attached to something else.
The OWL has a few more examples, so you can take a look there.