One Third or One-Third?

The hyphenated word one-third is the correct way to express a third of something. E.g., “He spent one-third of his salary on clothes.” Furthermore, the term one third as two words is incorrect, although some people write the term this way.

It is common practice to use a hyphen when writing the term one-third. This rule also applies to writing in APA Style and AP Style.

In addition, there is no variation between the UK and the US in the correct spelling version, and both countries use one-third rather than the two-word alternative.

However, although you will see people using one third as two words, it is a grammar error, and you should not use the term this way.

Please keep reading the rest of the page to learn more about using the term one-third in your writing.


The term one-third as a compound word is the correct way to express a third of something. Furthermore, this is the version you should use for academic or formal writing.

  • Each of my three children will get one-third of my estate.

Although some people sometimes write the term without the hyphen, it is more common to include it.

One Third

The term one third as two words with no hyphen is incorrect when referring to the fraction and is not the standard way of writing the term.

Nonetheless, people still write the term in this way, and most readers still understand it. However, you should not use this form in your writing and especially not for formal writing in APA or AP Style.

Furthermore, the word combination one third may crop up in other sentences unrelated to the fraction.

For example:

  • I want to read this one third because I haven’t read the first two books.


You should never write the term onethird as a single word without a hyphen as it is incorrect. It is not common to see people spelling the term like this.

Instead, when referring to a third of something, you should use one-third.

As shown in the following example:

  • Correct: This dress was discounted by one-third, so I bought it.
  • Incorrect: This dress was discounted by onethird, so I bought it.