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Large numbers that act like nouns

When you count things in French, standard numbers attach directly to the noun. You say trois chats (three cats) or mille euros (a thousand euros). But when you reach very large numbers or talk about groups of things, the rules change.

Words like million, milliard, douzaine, and centaine do not act like standard numbers. Instead, they act like regular nouns. Because they are nouns, they follow standard noun rules. They need an article in front of them, they take an s in the plural, and they cannot stand directly next to the object they are counting. You have to connect them with the word de (of).

Word list
millionA1 million un million de = a million of centainefA2 hundred une centaine de = about a hundred of douzainefB1 dozen une douzaine de = a dozen of milliardB2 billion un milliard de = a billion of quinzainefC1 fortnight une quinzaine de = about fifteen
Millions and billions

Let us look at the massive numbers first. If you want to say "one million" or "one billion", you must use an article: un million or un milliard. If you have two million, you add an s to make it plural: deux millions.

The biggest trap for English speakers is what happens next. In English, you say "two million people". In French, because these words are nouns, you are literally saying "two millions of people". You must use de (or d' before a vowel sound) between the quantity word and the noun.

Le gagnant va recevoir un million d'euros.
(The winner is going to receive a million euros.)
Deux millions de personnes vivent ici.
(Two million people live here.)
Ce projet coûte trois milliards de dollars.
(This project costs three billion dollars.)
Dozens and hundreds

French also has special nouns for standard groups or approximate quantities. You probably already know douzaine, which works just like "dozen" in English. French takes this concept further with words like centaine (about a hundred), dizaine (about ten), and millier (about a thousand).

Just like the massive numbers, these group words require an article, can be pluralized, and need de before the noun they quantify.

J'ai acheté une douzaine d'œufs.
(I bought a dozen eggs.)
Il y a une centaine de chaises dans la salle.
(There are about a hundred chairs in the room.)
Nous avons vu des centaines d'oiseaux.
(We saw hundreds of birds.)
The contrast with standard numbers

It is very important to contrast these noun-like numbers with standard numbers, especially mille (a thousand). Standard numbers never take an s in the plural and do not use de before the noun.

Compare deux mille personnes (two thousand people) with deux millions de personnes (two million people). The word mille acts like a normal adjective, while million acts like a noun.

J'ai invité trois mille personnes.
(I invited three thousand people.)
La ville a trois millions d'habitants.
(The city has three million inhabitants.)
Summary

Whenever you use a group word like a dozen or massive numbers like millions and billions, treat them like a container. You would say "a box of chocolates" in English. In French, apply the same logic: "a million of euros" or "a dozen of eggs". Remember the article, add an s for plurals, and always link them to the following noun with de.