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Noun-like quantity words followed by 'di'

When you count things in English, you usually place the number directly in front of the item, as in three cats or a million dollars. Sometimes you use a linking word, as in a pair of shoes. In Italian, basic numbers attach directly to the noun. However, certain larger numbers and grouping words act as true nouns. Because they are nouns, they cannot sit directly next to another noun. They need the preposition di to bridge the gap.

Words like milione, dozzina, and paio all follow this rule. If you want to say what you have a million or a dozen of, you must place di right after the quantity.

Word list
milionemA1 million un milione di = a million of paiomA2 pair un paio di = a pair of centinaioB1 hundred un centinaio di = about a hundred dozzinafB1 dozen una dozzina di = a dozen miliardomB2 billion un miliardo di = a billion of
Large round numbers

Basic Italian numbers like cento (a hundred) or mille (a thousand) are standard adjectives of quantity. You place them directly before the noun. But when you reach milione (million) and miliardo (billion), these words function as nouns. You have to say a million of something.

La città ha un milione di abitanti.
(The city has a million inhabitants.)
Il progetto costa un miliardo di euro.
(The project costs a billion euros.)

When you have multiple millions or billions, you use their plural forms: milioni and miliardi. They still require the preposition di to connect to the item.

Abbiamo speso due milioni di dollari.
(We spent two million dollars.)
Groups and approximations

This rule also applies to words that group items together. The word paio means a pair, and dozzina means a dozen. Just like in English, you need the equivalent of of to connect them to the object.

Ho comprato un paio di scarpe.
(I bought a pair of shoes.)
Vorrei una dozzina di uova, per favore.
(I would like a dozen eggs, please.)

Another very common quantity noun is centinaio. While cento means exactly one hundred and attaches directly to the noun, centinaio means roughly a hundred or a bunch of about a hundred. Because centinaio is a noun, it needs di.

C'era un centinaio di persone alla festa.
(There were about a hundred people at the party.)
Watch out for expanded numbers

There is a special exception to keep in mind. If a round million or billion is expanded by adding a smaller ordinary number right after it, the di is dropped. The presence of the smaller ordinary number interrupts the noun-like behavior of the million.

Il paese ha un milione trecentomila abitanti.
(The country has one million three hundred thousand inhabitants.)

Finally, remember that ordinary measurements work exactly like these numerical nouns. If you buy a kilogram of apples, you use the noun chilo followed by di, making it un chilo di mele. The logic is exactly the same as buying a dozen eggs.

Summary

When using noun-like quantities such as milione, miliardo, dozzina, centinaio, or paio, you must use the preposition di before the item being counted. Treat them exactly as you would treat words like a pair or a box in English.