Question:

I am often confused with the choice between “foot” and “feet.” How do I choose the right word in a particular situation? – Wéifēng,​ China
Answer:

Hello, Wéifēng, that is a great question!

For the unit of measurement, we often use the singular form even when we are talking about more than one foot. This can make things confusing for learners.

Luckily, there are three easy rules that can help:

When used as an adjective, we use “foot,” which is the singular form. Let’s hear some examples:

The children climbed a 15-foot tree.

Here, the adjective is “15-foot” and it describes the noun “tree.” Here’s another:

I have a 10-foot ladder that you can borrow.

Here, the adjective “10-foot” describes the noun “ladder.”

Notice that both 15-foot and 10-foot come before the noun and there is a hyphen between the words. A hyphen is needed when a unit of measurement acts as an adjective.

When used as a noun, we use the plural form: feet. Take a listen:

The tree is 15 feet high.

Here, the noun is “15 feet” and the adjective is “high.”

The ladder is 10 feet tall.

Here, the noun is “10 feet” and the adjective is “tall.” Notice that the noun form does not use a hyphen.

These two rules also apply to many other units of measurement.

Lastly, we usually use the singular form when talking about a person’s height. This is an exception to the plural noun rule in #2. Here’s an example:

I am 5-foot-6.

This is a common way of saying, “I am 5 feet 6 inches tall.”

However, when the person’s height is an exact number of feet --without inches -- we use the plural form.

I am 5 feet.

He is 6 feet tall.

The addition of “tall” is not required in everyday conversation.

Other Meanings

For all other meanings of “foot,” we use “foot” for the singular form and “feet” for the plural.