When should we use preposition "of" after number, amount or unit?

Question

This question may sound hard to English learners and too basic for English speakers. So, the placement of the question on the appropriated Stack Exchange website (ELL or English Native Speakers) can be arbitrary. The thing is: the use of the preposition "of" after numbers, amounts and measurement units always confuses me.

What is correct?

-3 kilograms OF oranges or 3 kilograms oranges?

-80% OF humidity or 80% humidity?

-At the spacing OF 3 meters or at the spacing 3 meters?

-100 kg OF N or 100 kg N?

-30 inches OF precipitation or 30 inches precipitation?

-50 meters OF height or 50 meters high?

According to Cambridge Dictionary preposition "of" is:

used after words or phrases expressing amount, number, or a particular unit

BUT, at the same time, I have found many scientific papers and websites not using the preposition "of" after such numerals. The Cambridge Dictionary does not say anything about cases of non-utilization of the preposition "of", when it is used and when it is okay.

Please, inform which of the pairs are correct. Especially if there is a general rule or common usage, you don't need to specify the correctness of each sentence. Just tell me if using "of" after the numerals/units is correct.

Answer 1

Although all of your examples illustrate exactly the same question, for clarity's sake we'll take your examples in order:

• 3 kilograms OF oranges or 3 kilograms oranges?

3 kilograms of oranges.

In natural speech, "of" would be included. You might see it omitted in a shopping list, but never in everyday speech or writing.

• 80% OF humidity or 80% humidity?

80% humidity

"Of" is omitted in common usage. The "of" is included, though, when we reverse the order of the terms. You might hear a meteorologist predict either: "Tomorrow, we will see humidity of 80%" or "Tomorrow we will see 80% humidity."

• At the spacing OF 3 meters or at the spacing 3 meters?

At the spacing of 3 meters

Here, normal usage will include "of." A list of instructions—for instance, to a carpenter—might read:

"Height 3 meters, Width 2 meters, Spacing 3 meters."

• 100 kg OF N or 100 kg N?

100 kg of N

See "oranges" above. In narrative use, we would include "of:" "Be sure to bring 100 kilos of N!" A list of constituents, though, might read:

200 kg Ammonium Perchloride
100 kg Nitrate of Gallodinium
150 kg Phosphorus

• 30 inches OF precipitation or 30 inches precipitation?

30 inches of precipitation

The "of" will almost always be included.

• 50 meters OF height or 50 meters high?

Either of these usages may be grammatically correct, depending upon context. "High" is an adjective, and "height" is a noun, though, so this example differs from the previous ones. In normal usage, though, you would almost never see "It has 50 meters of height." Instead, one would say either:

It is 50 meters high.

Or:

It has a height of 50 meters.

It's important to understand that you will frequently see technical or scientific writing, technical support documents, instructional manuals, and lists of things, which omit "of" for brevity's sake. There is no "rule" governing such usage except that "of" should only be omitted when its omission makes a statement unclear.

Answer 2

Of can be omitted if the item is part of a recipe or list of materials to make something.

I took 2 quarts milk, 1 pound oranges, and made something that tells me that I should give up cooking permanently.