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Hyphen in a Compound Adjective with Numbers

Updated on October 24, 2022Grammar
Colorful illustration of the post's title: "Hyphen in a Compound Adjective"

What is a compound adjective with numbers? In English grammar, we use hyphens when a noun and a number are used together as an adjective to describe another noun; such constructions are known as compound adjectives with numbers. For example, the phrases hundred-story building, 21-year-old actor, and one-hour drive are all compound adjectives with numbers, and they all use hyphens. 

While this is the simple answer, compound adjectives with numbers can get more complicated, such as when you need to identify a noun acting as an adjective or figure out when not to use a hyphen. Below, we explain everything you need to know about when to use hyphens in compound adjectives with numbers. 

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What are compound adjectives with numbers in grammar?

First, let’s answer the question, What are compound adjectives? If you’ve ever seen two words that act together as a single adjective, you’re already familiar with them. A compound adjective is a type of compound word that involves two or more words grouped together to make an adjective (a word that describes a noun). 

a fast-paced business

the well-known surgeon

a one-of-a-kind opportunity

As you may have noticed, the compound adjective examples above all use a hyphen. In English grammar, we use a hyphen with compound modifiers so we know which words go together and which don’t. Let’s look at an example:

small-animal doctor

The compound adjective small-animal tells us that this veterinarian specializes in small animals like mice or hamsters. However, without the hyphen, it looks like this: 

small animal doctor

Without the hyphen, it’s unclear whether small refers to the type of animal or to the doctor. This could mean a miniature-size vet! 

What about compound adjectives with numbers? If you’re using a noun with a number to describe another noun, that’s a compound adjective. That means it follows the same rules about hyphens, according to our grammar guide. 

ten-person maximum

eighteenth-century mentality

thirty-yard kick

Notice how each of the nouns in compound adjectives is singular, even though their numbers are greater than one. That brings us to a very important rule for compound adjectives with numbers: 

Always use the singular form of the noun in a compound adjective with a number. 

This may seem strange because there’s more than one of them (unless the modifying number is one). However, keep in mind that the noun is acting as an adjective, so it no longer follows the rules for nouns. That means the noun part of a compound adjective is always singular, whether the number is one or a billion. 

one-person tent

billion-tree forest

When do you use hyphens in compound adjectives with numbers? 

When the number/noun combination is an adjective

Any time you have a number and a noun that work together as an adjective, use a hyphen. As a group, they create a compound adjective and must follow the rules of other compound adjectives. You can usually identify compound adjectives because they come before the noun. 

fifty-dollar fine

forty-hour workweek

one-person show

When you’re using age as an adjective: #-year-old 

When describing someone’s age as an adjective using #-year-old, hyphens go between all the words. (Also, don’t forget that year is singular.) You can tell it’s an adjective because it comes before the noun. 

15-year-old teenager 

67-year-old senior

three-and-a-half-year-old program

Be careful though: When the phrase # years old comes after a noun, it does not use a hyphen. You also use the plural form of year unless they’re only one year old.  

My baby brother is only two years old

Fraction/noun combination as an adjective

Fractions, too, can become compound adjectives and use hyphens. 

half-million euros

quarter-mile race

half-baked scheme

Imperial abbreviations

While you can use hyphens with all written-out measurements in compound adjectives, abbreviations are another story. In compound adjectives, you can use hyphens only with abbreviated measurements if you’re using the imperial system of measurement (which is used in America). 

25-lb. dumbbell

6-ft. plank of wood

When do you not use hyphens in compound adjectives with numbers? 

When the number/noun combination is not an adjective

Above we mentioned that if the number/noun combination comes before the noun it describes, it uses a hyphen. Conversely, if the number/noun combination comes after the noun it describes, it doesn’t act like an adjective and does not use a hyphen. (Also, the noun is no longer singular.) That’s important to remember when writing and proofreading

fine of fifty dollars

workweek of forty hours

show starring one person

When age is not used as an adjective: # years old 

Just like the above rule, if the phrase # years old comes after the noun, do not use hyphens (and pluralize year). 

the teenager is 15 years old

a senior who’s 67 years old

This program will be three and a half years old soon. 

If the number is part of a proper noun or specific name

If your number/noun pair is already combined, such as in a name, you do not need hyphens. This includes not only artworks with numbers in their titles but also names of specific things, such as Grade 8 or Room 507.

She was diagnosed with Type 2 diabetes. 

The Interstate 65 pavement is full of potholes. 

Jasper Johns’s 0 through 9 paintings are fantastic. 

Metric abbreviations

While imperial abbreviations work with hyphens, abbreviations in the metric system do not.

20-m jog 

20 m jog 

However, if you spell out the measurement, hyphens are correct.

20-meter jog

Non-letter symbols

Do not use hyphens with non-letter symbols and abbreviations, such as degrees or currency marks. 

100-° F weather 

100°-F weather 

100° F weather 

 50-$ bill

$50 bill

Compound adjectives with numbers FAQs

What are compound adjectives with numbers?

In general, a compound adjective is two or more words working together to form an adjective to describe another noun. For example, fast-paced business. It’s quite common for one of those words to be a number, as in twenty-minute walk.

When do you use hyphens in compound adjectives with numbers? 

In compound adjectives with numbers, use a hyphen when the compound adjective comes before the noun it describes, such as thirty-story building. But don’t use a hyphen when it comes after the noun, such as building with thirty stories. Also use a hyphen for fractions (quarter-mile jog), abbreviations of imperial measurements (50-lb. dumbbell), and the phrase #-year-old if it comes before the noun it describes (15-year-old teenager).

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