In short, strictly, neither “uncompleted” nor “incompleted” is considered fully standard by most style guides — the correct, widely accepted word is simply “incomplete,” and if you need a verb form, “not completed” is the safer phrasing.
Why Both Forms Feel Wrong
“Un-” and “in-” are both negative prefixes in English, which is exactly why it’s unclear which one applies here — English doesn’t have a consistent rule for which prefix attaches to which root, so both forms get used informally despite neither being the dictionary-preferred choice.
The Standard Alternative
“Incomplete” is the standard adjective (“the task is incomplete”), and for describing an action that wasn’t finished, “not completed” (“the task was not completed”) is the cleanest, most universally accepted phrasing.
Is Either Form Ever Used?
“Uncompleted” appears occasionally in less formal writing and is more widely tolerated than “incompleted,” which is rarely found in edited, professional writing at all.
For Formal or Professional Writing
Stick with “incomplete” as an adjective or “was not completed” as a verb phrase — both forms are unambiguous and appear in every major style guide.
Frequently Asked Questions
Is “uncompleted” a real word? It appears in some dictionaries as informal, but “incomplete” is the preferred standard term.
Is “incompleted” ever correct? It’s considered nonstandard by nearly every major dictionary and style guide — avoid it in formal writing.
