The short answer
You can use an idiom in an essay only when the idiom matches the formality of the task and the reader shares its meaning. Most academic essays, reports and exam writing tasks expect neutral or formal English. In those tasks an informal idiom can cost marks even when the grammar is correct.
The question is not "is this idiom good English?" The question is "does this idiom belong in this text?"
Register: formal, neutral and informal
An idiom carries a level of formality, called its register. A dictionary entry for an idiom should tell you the register. Three levels matter for writing.
| Register | Where it fits | Example |
|---|---|---|
| Informal | Conversation, personal messages, dialogue in a story | kick the bucket, hit the sack, grab a bite |
| Neutral | Most writing, including many essays and articles | make a decision, take part, keep in touch |
| Formal | Reports, academic papers, official letters | in accordance with, on behalf of, with regard to |
"Kick the bucket" means "die". It is informal and it can sound flippant. In an essay about a person's life, died or passed away is safer. In a comic story, the idiom may be the better choice.
When idioms help
- Narrative writing. A character who says "I'm over the moon" sounds like a person, not a textbook.
- Opinion essays with a personal voice. One well-chosen neutral idiom can make a point memorable.
- Speaking tests. IELTS, TOEFL and Cambridge speaking tasks reward natural language, including some idioms.
- Journalism and blogs. A headline often uses an idiom because it is short and vivid.
When idioms hurt
- Academic essays. An idiom can look too casual and can hide a weak argument. "The results were a piece of cake" says less than "The results were easy to reproduce".
- Exam writing tasks that ask for formal language. The mark scheme rewards accurate register. An informal idiom can lower the score.
- International readers. An idiom tied to one culture may be unclear to a reader from another country.
- Technical and legal writing. These texts need exact meaning. An idiom adds ambiguity.
Four tests before you use an idiom
- Check the label. Look the idiom up and read its formality label and its example sentence. If the label says informal, do not put it in a formal essay.
- Check the reader. Ask whether a reader outside your culture would understand the image. If not, say it plainly.
- Check the fit. Read the sentence aloud. An idiom that clashes with the tone of the paragraph pulls the reader out of the argument.
- Check the plain version. Write the same idea without the idiom. If the plain version is clearer and shorter, use it.
Safer replacements
| Informal idiom | Meaning | Neutral or formal version |
|---|---|---|
| a piece of cake | very easy | straightforward, simple to do |
| cost an arm and a leg | very expensive | expensive, costly |
| let the cat out of the bag | reveal a secret | reveal the plan, disclose the information |
| once in a blue moon | very rarely | rarely, infrequently |
| get the hang of it | learn how to do something | become familiar with it, learn the method |
Common mistakes
| Mistake | Why it is a problem | Fix |
|---|---|---|
| Changing the words of a fixed idiom | The phrase stops being an idiom and sounds like an error. | Use the wording in the dictionary example. |
| Mixing two idioms | The image becomes confused, for example "a piece of the pie in the sky". | Choose one idiom and finish it. |
| Using five idioms in one paragraph | The writing sounds forced and the meaning gets weaker. | Keep one idiom and state the rest plainly. |
| Translating an idiom from your first language | The result is often not English, even when each word is correct. | Check the English dictionary entry before you use it. |
Try the quiz
Answer the questions in writing first. Then open the answer key.
- Is kick the bucket suitable for a formal report? Why?
- Replace a piece of cake in a formal sentence about an exam.
- Name one writing task where an informal idiom can help.
- Fix the mixed idiom: It was a piece of the pie in the sky.
- True or false: a good dictionary entry for an idiom shows its formality label.
- Rewrite plainly: The meeting happens once in a blue moon.
Show all answers
- No. It is informal and it can sound flippant about death.
- Example: The exam was straightforward. Keep the meaning and remove the idiom.
- A narrative, a speaking test or an informal blog post.
- Choose one image: It was a piece of cake, or It was pie in the sky.
- True. The label tells you which settings fit.
- The meeting happens rarely.