One Third or One-third? Understanding Fractions in American English

When it comes to fractions, even experienced writers and editors sometimes pause before typing one third or one-third. In American English, the right usage depends on how the fraction fits into the sentence. You should hyphenate it when it comes before a noun—like “a one-third share”—because the hyphen keeps the idea together. But when it stands alone, it’s fine to skip the hyphen and simply write “one third.” I’ve seen this rule trip people up in emails, blog posts, and even formal reports, so keeping a small quick-reference note helps a lot.

Many style guides—including major ones—treat this topic with slight differences, but they all agree that clarity, correctness, and consistency matter most. While drafting or preparing your writing, always check your grammar, punctuation, and formatting to maintain smooth comprehension. A simple table of examples or tips can offer practical guidance when you need to write confidently. Over time, I’ve found this attention to language details incredibly valuable for keeping my style polished and professional.

What Does “One Third” Mean?

When you see one third, you’re dealing with a fraction that represents dividing something into three equal parts and selecting one of those parts. In plain language, if you sliced a pizza into three equal slices and you took one, you have one third of the pizza.
Here are a few everyday examples:

• One third of the day has passed if 8 of 24 hours are gone.
• One third of Americans might prefer one option over another—so 33.3%.
• If you read one third of a book, you’ve read roughly 33.33% of it.

In each case the fraction names a portion of a whole. It functions as a noun (one third) or part of a noun phrase. The meaning is clear: one piece out of three equal pieces.

The Grammar Behind Fractions in English

Fractions like “one third” can play different grammatical roles: as nouns, as adjectives (modifiers) or sometimes adverbs. How they function affects whether you hyphenate or not.

Fraction as a noun

When you use a fraction to name a part of something, it behaves like a noun. Example:

“One third of the class was late.”

Here one third is the subject of the sentence—it doesn’t modify a noun, it is the noun phrase.

Fraction as an adjective

When the fraction comes before a noun and modifies it, it acts like an adjective (compound modifier). Example:

“A one-third share of the profits was distributed.”

Here one-third describes the share. Because it modifies a noun, many style guides favour hyphenation for clarity.

Here’s a table summarizing:

“One Third” vs. “One-third”: The Key Difference

So what’s the real difference between one third and one-third?

• Use one-third (hyphenated) when the fraction acts as an adjective modifying a noun: a one-third increase, the one-third portion, her one-third share.
• Use one third (no hyphen) when the fraction is used as a noun or part of a noun phrase: One third of the apples were rotten.

Quick tip: If the fraction comes before a noun and modifies it, hyphenate. If it stands alone as a noun phrase or follows a verb, drop the hyphen.

Let’s look at example sentences:

• Correct: “A one-third discount applies.”
• Correct: “One third of the participants dropped out.”
• Incorrect: “The one third discount applies.” (should hyphenate)
• Incorrect: “She ate a one-third of the cake.” (since it’s a noun, better: “She ate one third of the cake.”)

This rule isn’t carved in stone for all writing, but it reflects the preferred usage in many style guides and professional writing contexts.

Common Examples in Everyday Writing

Here are real-life cases you’ll encounter in newspapers, blogs, academic writing, and corporate reports:

• One-third: A one-third increase, a one-third chance, a two-thirds majority.
• One third: One third of Americans support the measure; they completed one third of the work.
• In numeric form: 1/3 of the budget was spent; 2/3 of the respondents agreed.
• Mixed forms: “Two-thirds of the crowd cheered,” “He finished one third of the race.”

Mini-exercise
Decide which form is correct in each sentence:

1. She gave him one third of the cookies.
2. The project experienced a one-third reduction in costs.
3. Two-thirds of the votes were counted.
4. Nearly one third of the audience left early.

Answers:

1. one third (noun)
2. one-third (adjective)
3. two-thirds (adjective)
4. one third (noun)

Using lots of examples like this helps internalize the rule.

American English vs. British English Usage

You may wonder: do Americans and Brits do this differently? Yes—there are subtle variations. In American English, style guides like the The Associated Press Stylebook (AP) more explicitly recommend hyphenation for adjectival fractions. For example: “two-thirds” when used as a modifier. (Writing Explained) British English may be a bit more relaxed: you’ll find “one third” without hyphen in some contexts even when it comes before a noun.

Here’s a comparative table:

If you’re writing for an American audience or using U.S. style guides, hyphenation before nouns is safer for clarity.

Style Guide Rules (AP, Chicago, MLA, APA)

Several major style guides weigh in:

Takeaway: While specifics vary, the shared principle is clear—hyphenate when the fraction is used as a descriptive modifier, less so when it stands alone.

Hyphenation Rules Simplified

Let’s simplify the hyphenation rules into memorable tips you can apply quickly:

•  Use a hyphen when the fraction appears before a noun and modifies it: one-third portion.
•  No hyphen when the fraction appears after a verb and functions as a noun phrase: the portion was one third.
•  Keep hyphenation consistent in formal writing.
•  When in doubt, ask: Is the fraction acting as a descriptor (modifier) or naming a quantity?
•  Visual mnemonic:

Before noun → Hyphenate

After verb/noun → No hyphen

Correct vs. Incorrect:

•  A one-third increase in sales.
•  A one third increase in sales.
•  The increase was one third.
•  The increase was one-third.

By using these simple heuristics, your writing becomes more polished and error-resistant.

Fractions in Numbers and Words

Beyond hyphens there’s the matter of when to use words and when to use numerals. For example, should you write “one third” or “1/3”? Most style guides suggest:

• Spell out fractions less than one when words are preferred, especially in narrative text: one-third of the class. (www)
• Use numerals when precision matters, especially in scientific or data contexts: 1/3 of respondents.

Example guidance:

• “1/3 of the sample responded.” (data-driven context)
• “About one-third of the audience laughed.” (narrative context)
• In headlines: numerical form may be acceptable for brevity.

So adapt your form based on context, audience, and tone.

Common Mistakes and How to Avoid Them

Writers often stumble over fractions. Here are common errors and how to steer clear:

Mistakes

• Writing “one-third” when it’s used as a noun after the verb: The cake was cut into one-third pieces.
• Omitting the hyphen when the fraction modifies a noun: a one third chance.
• Inconsistently using hyphens within the same document: mixing “one-third” and “one third” unpredictably.
• Using “1/3rd” instead of “one third” or “1/3”—the “rd” is nonstandard and awkward.

How to avoid them

• Review each sentence and ask: Is this fraction modifying a noun (adjective) or naming a quantity (noun)?
• Apply the hyphenation rule consistently.
• Use style guides or a house-style document to maintain uniform usage.
• Proofread specifically for fractions and hyphens—these details matter.
• Include a quick reference note for yourself: fraction + noun → hyphen; otherwise, no hyphen.

Related Fraction Expressions

The hyphenation rules for “one third” apply similarly to other fractions. Consider these common ones:

Key point: The pattern stays consistent—hyphenate when the fraction precedes and modifies a noun; don’t when it’s used as a noun phrase.

Read More:“In Efforts To” vs. “In An Effort To” – Difference & Meaning

Idioms and Figurative Use of Fractions

Fractions show up in idioms and expressions too—so the hyphen-rule still matters. Examples:

• “He gave it his one-third effort.” (modifying “effort”)
• “Two‐thirds of success is showing up.” (modifying “success,” though you might see non-hyphen use)
• “We are one-half way there.” (modifying “way”)

Even in figurative speech, clarity counts. Hyphenation can help readers parse meaning faster—it ties the words together visually and conceptually. That said, style and context may allow flexibility.

Visual Guide: When to Use Hyphens

Here’s a simple diagram/checklist to keep handy:

[ Is the fraction before a noun and modifying it? ]

      ↓ Yes  → Hyphenate (one-third outcome)

      ↓ No   → Don’t hyphenate (one third of the outcome)

Checklist:

• Before a noun → Hyphenate
• After a verb or as stand-alone noun → No hyphen
• Consistent from sentence to sentence? → Yes → Good
• Audience and style-guide aligned? → Yes → Better

You can also create a style-sheet note for your writing: “Fractions used adjectivally get hyphens; otherwise not.”

Quick Recap Table

Here’s a handy summary you can refer back to:

Keep this table near you when writing—especially in reports or long pieces where consistency matters.

Conclusion

Mastering how to write one third or one-third might seem like a small detail, but it can make a big difference in your writing. Knowing when to hyphenate ensures clarity, correctness, and consistency across all types of documents, whether you’re drafting a report, an email, or a blog post. Follow the style guides, use quick-reference tables for examples, and you’ll soon write with more confidence. Remember, good grammar and thoughtful punctuation show attention to detail—something every strong writer and editor values.

FAQs

1. Should I write “one third” or “one-third”?

Use one-third with a hyphen when it comes before a noun (e.g., one-third portion). Use one third without a hyphen when it stands alone.

2. Do all style guides agree on this rule?

Most major style guides in American English treat it similarly, though some have slight differences. Always check your preferred guide for guidance.

3. Why does it matter if I use a hyphen or not?

Because hyphenation affects clarity and comprehension. A missing hyphen can make your writing look inconsistent or confusing.

4. Can I apply this rule to other fractions?

Yes. The same rule applies to other fractions, such as two-thirds or three-quarters. Always hyphenate when they come before a noun.

5. What’s an easy way to remember this?

Keep a quick-reference list or table of examples handy. Over time, you’ll naturally follow the right usage without thinking twice.

Bella Rae