Unfeasible or Infeasible: A Simple Guide to Using the Right Word

Unfeasible and infeasible both mean “not feasible,” but infeasible is more common in formal writing.

You are writing a project report. You need to explain that a plan will not work. You type “unfeasible.” Then you pause. Should it be “infeasible” instead? Both words look correct. Both feel formal. But which one should you actually use?

This question comes up often in business writing, engineering reports, and academic papers. People search for this pair because both words appear in respected sources, both pass spell check, and both mean almost the same thing. That similarity creates real doubt, especially when a document needs to sound polished and professional.

This confusion matters more than it seems. In technical fields, choosing the standard term signals that you know the field. In business writing, using the less common word can make a document feel slightly off, even if no rule is broken.

This article gives you a clear answer first. Then it covers where these words came from, how they are used today, and which one fits your writing best. By the end, you will use “unfeasible” or “infeasible” with full confidence.

Unfeasible or Infeasible: Quick Answer

Both words are correct, and both mean “not possible” or “not practical to do.” They are close synonyms.

Infeasible is the more common choice today. It shows up more often in business, science, engineering, and technical writing.

Unfeasible is also correct and widely understood. It appears slightly more in general and everyday writing.

Examples: “The engineering team called the design infeasible due to cost limits.” “Given the short deadline, the plan felt unfeasible.”

You can use either word in most sentences, but infeasible is the safer default for formal or technical work.

The Origin of Unfeasible or Infeasible

Both words come from the same root word, “feasible,” which entered English in the 1400s from the Old French word “faisable,” meaning “able to be done.” That word traces back to the Latin verb “facere,” meaning “to make” or “to do.”

To create the negative form, English writers added a prefix. Two different prefixes were used over time, which is why we ended up with two competing words instead of one.

“Un” is a native English prefix, used to reverse the meaning of many adjectives, such as “unhappy” or “unclear.” Writers naturally applied it to “feasible” to form “unfeasible.”

“In” is a Latin based prefix, also used to reverse meaning, seen in words like “incomplete” or “inaccurate.” Since “feasible” already came from Latin roots, many writers applied the Latin style prefix instead, forming “infeasible.”

Both versions developed side by side in English. Neither one is a spelling error of the other. They are two separate word formations built from the same base word.

British English vs American English Spelling

Unlike true regional spelling pairs such as “colour” and “color,” unfeasible and infeasible are not split cleanly between British and American English. Both words are used and understood in the US, the UK, Canada, Australia, and other English speaking regions.

That said, usage patterns differ slightly by context rather than by country. Infeasible appears more often in technical, scientific, legal, and business writing across all English speaking regions. Unfeasible appears more often in general conversation and everyday writing, with a slightly stronger presence in British English style guides.

Word	US Usage	UK Usage	Common Context
Infeasible	Common	Common	Technical, scientific, legal, business
Unfeasible	Less common	Somewhat more common	General, everyday, conversational

The real difference here is formality and field, not geography.

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Which Spelling Should You Use?

Your choice should depend on your audience, your topic, and how formal your writing needs to be.

For technical, scientific, engineering, or business writing, choose infeasible. It is the standard term in reports, research papers, and professional documents worldwide.

For everyday writing, casual emails, or general conversation, either word works, though unfeasible can sound a touch more natural in relaxed British style writing.

For UK or Commonwealth audiences, both words are accepted, but many British style guides and dictionaries still list unfeasible as a valid and common alternative.

For global or mixed audiences, infeasible is the safer, more universally recognized choice, since it dominates in academic and professional writing worldwide.

Common Mistakes with Unfeasible or Infeasible

1. Assuming one word is incorrect. Both words are valid English words with the same core meaning.
2. Mixing both words in the same document. Using “infeasible” in one paragraph and “unfeasible” in another can look inconsistent in formal writing.
3. Overusing the rarer word in technical fields. Choosing unfeasible repeatedly in a scientific report can feel slightly out of place compared to the field standard, infeasible.
4. Confusing these words with “not feasible.” All three options carry the same meaning, but many style guides prefer the simpler phrase “not feasible” in plain writing.
5. Forgetting audience expectations. A casual audience may find infeasible overly formal, while a technical audience may find unfeasible unusual.

Unfeasible or Infeasible in Everyday Examples

In emails: “After reviewing the budget, this proposal is infeasible for this quarter.” “Honestly, finishing this by Friday feels unfeasible.”

In news headlines: “Experts Call New Housing Plan Economically Infeasible.” “Officials Say Rapid Rollout Is Unfeasible Without More Funding.”

In social media posts: “This deadline is completely infeasible. 😩 #WorkLife” “Trying to finish a marathon in a week? Totally unfeasible. 🏃”

In formal writing such as reports and research papers: “The committee concluded that the proposed timeline was infeasible given current resources.” “Field testing showed that the original design was unfeasible under real world conditions.”

Unfeasible or Infeasible: Google Trends & Usage Data

Search and usage data show a clear global lean toward infeasible, especially in business, technology, and academic content, where the word appears far more often in reports, whitepapers, and research writing. This pattern holds true across the US, UK, Canada, Australia, and other major English speaking regions.

Unfeasible still receives steady search interest, largely from people checking whether the word is correct or comparing it directly to infeasible. Interest in this word tends to rise in general writing, opinion pieces, and everyday conversation rather than in technical documents.

Because the difference here is more about tone and field than region, usage data reflects topic and formality far more strongly than country of origin.

Keyword Variation Comparison

Variation	Meaning	Common Context
Infeasible	Not possible or practical	Technical, scientific, business, legal
Unfeasible	Not possible or practical	General, everyday, conversational
Not feasible	Not possible or practical	Plain language, simple writing
Feasibility	The quality of being possible or practical	Reports, studies, planning documents
Infeasibility	The state of being infeasible	Formal analysis, technical writing

Frequently Asked Questions

1. Is unfeasible or infeasible correct? Both words are correct English words with the same basic meaning, “not possible or practical.”
2. Which word is more common? Infeasible is more common overall, especially in business, science, and technical writing.
3. Is this a British versus American spelling issue? Not exactly. Both words are used in every major English speaking region. The real difference is formality and field, not geography.
4. Can I use these words interchangeably? Yes, in most cases. For technical or professional writing, infeasible is the safer, more standard choice.
5. Which word do dictionaries prefer? Most major dictionaries list both words, though many usage guides note infeasible as the more widely accepted term in formal writing.
6. Is unfeasible considered incorrect or old fashioned? No. It is still valid modern English, though it appears less often than infeasible in professional documents.
7. Should I use “not feasible” instead of either word? This is a solid option, especially for plain language writing, since it avoids any doubt about which form to choose.

Conclusion

Unfeasible and infeasible both mean the same thing, “not possible or practical,” and both are grammatically correct. They come from the same root word, feasible, but took different paths through English by using two different negative prefixes, one native English and one Latin based.

There is no strict British versus American divide here. Instead, the choice comes down to formality and field. Infeasible dominates technical, scientific, and business writing across every major English speaking region. Unfeasible remains valid and common in general, everyday writing, especially in more relaxed or conversational contexts.

For safest results, default to infeasible in professional or technical documents, and feel free to use unfeasible in casual writing where it fits naturally. Whichever word you choose, stay consistent within the same document, and consider the simpler phrase “not feasible” whenever plain, clear language is the priority.