I heard a caller say this on the radio during a gardening show this morning.

Problem:
The noun “foot” is incorrect in the caller’s statement.

Explanation:
The caller was describing the height of a tree to the gardening-show host.

The caller said, “It’s about 25 foot tall.”

It is correct to use the singular noun “foot” when hyphenating it with a number to form a compound modifier of another noun.

Here are some examples in which a number is hyphenated with a singular noun such as “foot” to form a compound modifier of the subsequent noun:

• 25-foot wave
• 24-inch level
• 5-mile run
• 3-layer fabric
• 100-foot hose

However, the word “tall” is not a noun except in a vernacular specifically related to describing clothing, so “It’s about 25-foot tall.”, in which we put a hyphen between “25” and “foot”, is not the solution to the problem in the hyphen-free sentence “It’s about 25 foot tall.”

The only acceptable value for “X” in the hyphen-free sentence “It’s about X foot tall.” is a value of one or less, as in:

• “It’s about one foot tall.”
• “It’s about 0.7 foot tall.”
• “It’s about 0.1 foot tall.”

The reason for this is that the word “tall” in all of these sentences is an adjective that indicates the directional dimension to which the speaker is referring.

In other words, all of these sentences are answers to the question “How tall is it?”, and the word “tall” can be dropped from the preceding three sentences without harming the meaning:

• “It’s about one foot.”
• “It’s about 0.7 foot.”
• “It’s about 0.1 foot.”

Given that “It’s about 25 foot.” would be an incorrectly formed sentence because “25” should be followed by a plural noun instead of a singular noun, we arrive at the solution to the problem.

Solution:
“It’s about 25 feet tall.”