It is easy to confuse the difference between fewer and less because they are very similar in meaning. This post will show you how to use each of those words correctly.
The Elements of Style perfectly explains the difference between fewer and less: “Less refers to quantity, fewer to number.”
For example: “My sister wants to drink less coffee.” In this example, “less coffee” refers to quantity, not number (we aren’t counting actual coffee or cups of coffee).
For example: “My sister drank fewer cups of coffee today than she drank yesterday.” In this example, “fewer cups” refers to the actual number of cups of coffee that was drunk.
The above explanation is the traditional usage for fewer and less. Grammar girl offers a different rule: “The Singular vs Plural rule.”
Use less when using singular nouns and use fewer when using plural nouns.
Let’s go back to the examples above:
For example: “My sister wants to drink less coffee.” In this example, coffee is not only used quantitatively it is also a singular noun.
For example: “My sister drank fewer cups of coffee today than she drank yesterday.” In this example, the actual cups of coffee can be counted and cups is a plural noun.
I like Grammar Girl’s rule because it covers a lot of exceptions that are not covered by the traditional fewer/less rule such as money, time, etc.
For example: “Jeff made $300 less income this month.” Technically, $300 (three hundred dollars) is plural but we usually treat money as a singular noun. Thus making less the correct choice.
The same is true for time, distance, and even weight. We often treat them as singular nouns so it is correct to use less when referring to these concepts.
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