Both "unfeasible" and "infeasible" are words according to spell-check, and they appear have similar dictionary definitions. But what is the difference between the two words? Is one more acceptable to use than the other?

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    Here is an interesting piece of this issue: economist.com/blogs/johnson/2012/07/variation
    – user66974
    Commented Nov 9, 2014 at 7:39
  • I'm not sure of the difference either, but what I am sure of is that, in "The infeasibility of the project became apparent", infeasibility is a noun, not an adjective, and that in "Completion of the project within the timeframe became unfeasible," unfeasible is an adjective following 'become' as they do following'get''BE' 'seem' etc. ie, it is not an adverb as claimed.
    – vincent
    Commented Nov 15, 2016 at 10:37

2 Answers 2


Jacking the Link From The Comment (Thanks Josh61), It seems that the words are interchangeable, and it matters not which you choose to use, and is completely down to preference.

One interesting thing to note is that the variant with the "un" prefix was the most common, until the 1970's when the "in" prefix shot past it by a long way. It would be interesting to know the reason for this, but I guess that is a question for another time.

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    – Keavon
    Commented Nov 11, 2014 at 20:24
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    Interesting! The same change took place in both American and British English, which is not always the case. Commented Jun 22, 2020 at 7:49

It seems preferable to use infeasible as an adjective, as in, "the infeasibility of the project became apparent", and unfeasible as an adverb, as in, "completion of the project within the timeframe became unfeasible."

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    Hmm, what country is this? The second example still sounds weird to the ears over here.
    – Pacerier
    Commented Mar 3, 2016 at 17:37
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    “Infeasibility” is a noun and “unfeasible”, as nan gray notes, is an adjective
    – herisson
    Commented Jan 15, 2018 at 7:32
  • @sumelic "Infeasibility" is NOT a noun in this example, it is an adjective, descriptive of the noun, "project" as noted by Kiki.
    – Wallace
    Commented Jan 6, 2019 at 13:18
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    @Wallace: “-ity” words are (abstract) nouns, even when they’re used to describe other nouns.
    – herisson
    Commented Jan 6, 2019 at 15:54
  • "It seems"? Source?
    – Clonkex
    Commented Oct 4, 2022 at 22:23

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