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As a matter of objective grammar rules, without the strictness of a specific style guide or geographic vernacular, is the following sentence grammatically incorrect:

"The book is 20 pages."

?

Why or why not?

What about the following similar sentences:

"The race is 13 miles."

"The building is 20 floors."

"The schoolchildren in the class are all 11 years."

"The movie is two hours."

Are all those all grammatically okay? All incorrect? Some okay and some not okay?

Keep in mind, I'm not asking how you would write those sentences or asking if there are clearer and better ways to write those sentences. Rather, I am asking simply whether they are objectively grammatically incorrect as written.

Without being an expert or using google, I’d say they’re missing an adjective.

The book is 20 pages long.

The building is 20 floors high.

I think they are all incorrect for two reasons. First, the verb "is" states an equivalence. A race is not the same as a mile, and a book is not a page or even a collection of pages. Second, unless you state a unit of measurement, you are making the reader work too hard (not nice), and some of the sentences could be misunderstood. Are all of the children 11 years old? Or are they 11 years behind in their math skills?

The sentence you gave is considered an independent clause since no extra words are needed to complete the sentence. It is structured correctly since it could be an introduction to what will be written next. Ex: The book is 20 pages. On the first 5 pages, the author introduced...etc.
Formal writing requires a sentence with a subject and a verb.

Based on grammatical rules, the sentence can stand out on its own. So yes, it is grammatically correct.

But, I would not leave it like this because there is nothing to describe the pages. Are the pages wet? Are the pages dry? Are the pages short? Are the pages long? If one were to use sentences like these in an essay or blog, they can make a blog post boring fast.

Depending on the case, I think it's a language trend from a certain speech community. And if the author included all those sentences in a book, then it's not a mistake but rather the way some peolpe speak