Infeasible or Unfeasible: Which Is Correct?

Both infeasible and unfeasible are correct. They mean exactly the same thing — not capable of being done — and both are recorded in major dictionaries. Merriam-Webster gives unfeasible a full dictionary entry (“not capable of being done or carried out: not feasible”), and Cambridge lists the two as cross-referenced synonyms. Neither is a misspelling.

What Do They Mean?

There is no daylight between the definitions. Cambridge defines infeasible as “not able to be achieved” and cross-references it to unfeasible, which it defines as “not feasible (= able to be done or achieved)” — adding “(also infeasible)” to the entry. Merriam-Webster defines unfeasible as “not capable of being done or carried out: not feasible” and lists infeasible among its synonyms, and vice versa. Cambridge’s thesaurus groups them together under “not able to be achieved,” alongside impossible, unachievable, unattainable, unworkable, and impracticable.

Both describe plans, ideas, or proposals that cannot be carried out in practice — usually because of cost, time, physics, or logistics rather than pure impossibility:

• “The company promised a fully self-driving car in a year, but engineers say it is technically infeasible.” (Cambridge’s example)
• “It would be unfeasible to try to ban all petrol vehicles at the moment.” (Cambridge’s example)
• “The solution — cutting spending or raising taxes — is politically unfeasible, especially with elections approaching.” (Forbes, via Merriam-Webster)
• “Nearly two-thirds of the approved homes were determined to be financially unfeasible.” (Mercury News, via Merriam-Webster)

The Surprising History: Unfeasible Came First

Most people who question this pair assume unfeasible is the mistake — an awkward coinage that should have been infeasible. The history runs the other way. Merriam-Webster dates the first known use of unfeasible to 1527, in the meaning defined above. It was the more common of the two words for centuries.

Infeasible only overtook it in the late 1970s — in both American and British usage, according to usage references. Since then, infeasible has become the somewhat more common form, particularly in American English and in technical writing. But unfeasible never became wrong; it simply became the less frequent twin.

The two prefixes tell their own story. Feasible comes from Old French faisable (“doable”), from faire, to do — ultimately from Latin facere. In- is the Latin negative prefix, so infeasible is etymologically “pure”: Latin prefix on a Latin-derived root, cognate with French infaisable. Un- is the Germanic negative prefix, making unfeasible a hybrid — a Germanic prefix on a French-derived word. English is full of such hybrids (unhappy, unable), so there is nothing illegitimate about it.

Pronunciation

Cambridge gives UK /ɪnˈfiː.zə.bəl/ for infeasible and UK /ʌnˈfiː.zə.bəl/ for unfeasible (US pronunciations match). The stress falls on the second syllable in both: in-FEE-zuh-bul, un-FEE-zuh-bul.

Which One Should You Use?

Since both are correct, the choice comes down to context and consistency:

• Technical, scientific, and mathematical writing: prefer infeasible. It is the established term of art — an “infeasible solution” in optimization is one where no combination of variables can satisfy all the constraints. Using unfeasible here would look out of place to specialists.
• Business, policy, and general writing: either is acceptable. Infeasible is currently the more common choice in American English; unfeasible remains perfectly natural, especially in British English.
• Any single document: pick one and stay with it. Alternating between the two draws attention to the variation for no reason.

If you want the option that raises the fewest eyebrows across all audiences, choose infeasible — it is the more frequent form today and the technical standard. But do not “correct” someone else’s unfeasible: it is a real word with nearly five centuries of use behind it.

Examples in Sentences

• Building a bridge here is infeasible given the budget.
• Building a bridge here is unfeasible given the budget.
• The timeline the client proposed is technically infeasible.
• Holding the event outdoors in January would be unfeasible.
• The optimization returned no solution — the problem as stated is infeasible.
• They dismissed the idea as economically unfeasible.

Each sentence works with either word. The choice changes the register slightly, never the meaning.

Correct vs Incorrect Usage

Sentence	Verdict
The plan is infeasible.	✓ Correct
The plan is unfeasible.	✓ Correct (older word, still standard)
The plan is infeasible, so we need an unfeasible alternative.	✗ Inconsistent — pick one per document
The plan is disfeasible.	✗ Not a word
The plan is unfeasable.	✗ Misspelling — the root is feasible
The problem is infeasible to solve.	~ Grammatical but clumsy — prefer “impossible to solve” or “it is infeasible to solve the problem”

Common Mistakes to Avoid

1. Assuming “unfeasible” is wrong

The single most common error with this pair. Unfeasible has a full entry in Merriam-Webster, a full entry in Cambridge, and a first known use in 1527. Flagging it as a mistake says more about the flagger than the word.

2. Mixing the two in one document

Because both are correct, inconsistency is the real fault. A report that calls a plan “infeasible” on page one and “unfeasible” on page three looks unedited, even though neither instance is wrong on its own.

3. Using either to mean “unreasonable” or “unfair”

Both words describe practical impossibility — something that cannot be done or carried out — not moral judgment. A proposal can be infeasible (impossible to execute) without being unreasonable (unfair or illogical). Keep the meanings separate.

4. Misspelling the root

The root is feasible, spelled -ible, not -able. Unfeasable and infeasable are misspellings. The -ible ending comes through French faisable, and English kept the i.

Frequently Asked Questions

Is “unfeasible” a real word?

Yes. Merriam-Webster gives it a full dictionary definition — “not capable of being done or carried out: not feasible” — and dates it to 1527. Cambridge lists it as a standard adjective synonymous with infeasible.

Is “infeasible” American and “unfeasible” British?

Not exactly. Both words are used on both sides of the Atlantic. Infeasible is somewhat more common in current American usage, and unfeasible retains a stronger presence in British English, but the divide is one of frequency and register, not of correctness by region.

What is the difference in meaning?

None. Dictionaries define each in terms of the other. Any perceived difference is a matter of personal habit or field convention (mathematics strongly prefers infeasible).

Why do the two forms exist?

English has two productive negative prefixes: Germanic un- and Latin in-. Both were applied to feasible in the 1500s, and both stuck. Similar doublets exist elsewhere in the language (unhuman/inhuman being a famous pair where the meanings did drift apart — but here they did not).

Is “not feasible” better than either?

Often, yes — in plain writing, “not feasible” is the clearest option and sidesteps the choice entirely. But in formal or technical prose, the single-word adjectives are standard and expected.

Close Synonyms: Choosing Between Them

Cambridge’s thesaurus groups infeasible and unfeasible with several near-synonyms, and choosing among them is a matter of shade rather than correctness:

• Impossible — the strongest term. Use it when something cannot happen under any circumstances, not merely when it is impractical. An infeasible plan might become feasible with more money; an impossible one cannot.
• Unachievable / unattainable — emphasize the goal rather than the plan. Goals are unattainable; methods are infeasible. “An unattainable target” but “an infeasible schedule.”
• Unworkable — the closest synonym, and often interchangeable with both words. It leans slightly toward plans and proposals: “an unworkable compromise.”
• Impracticable — formal, and the most precise synonym. Cambridge’s usage note warns not to confuse it with impractical: “impracticable describes things that cannot or should not be done,” while “impractical is used to describe things that can be done or achieved successfully, though not easily.” An impracticable scheme cannot be executed; an unfeasible one merely faces difficulty.

In most everyday writing, infeasible/unfeasible and unworkable cover the same ground. Reach for impracticable in formal prose, impossible when you mean absolute impossibility, and plain “not feasible” when clarity matters more than elegance.

The Takeaway

Infeasible and unfeasible are both correct, both mean “not able to be done,” and both have dictionary entries to prove it. Unfeasible is the elder, dating to 1527; infeasible is the current frequency leader and the technical standard. Choose the one that fits your context, use it consistently, and never let anyone tell you unfeasible isn’t a word.

Sources: Merriam-Webster — “unfeasible”; Cambridge Thesaurus — “infeasible”