French numbers from 70 to 99 famously abandon the clean base-ten pattern used from 20-69 and switch to a vigesimal (base-20) counting logic left over from an older number system, which makes this range the single hardest part of French numbers for English speakers. Seventy is 'soixante-dix', literally 'sixty-ten' — you keep counting up from soixante (60) as if seventy didn't have its own word: 71 is soixante et onze ('sixty and eleven'), 72 is soixante-douze ('sixty-twelve'), all the way to 79, soixante-dix-neuf ('sixty-nineteen'). Note that 71 uses 'et onze' (with 'et', no hyphen), breaking the pattern you'd expect from 72-79, which use plain hyphens.
Eighty is 'quatre-vingts', literally 'four twenties' (4 × 20 = 80) — a direct vigesimal calculation. Note the -s on vingts only appears for exactly 80; as soon as you add anything after it, the -s drops: quatre-vingt-un (81, and note there is no 'et' before 'un' here, unlike soixante et onze), quatre-vingt-deux (82), up to quatre-vingt-neuf (89). Ninety continues the same logic: quatre-vingt-dix, literally 'four-twenty-ten' (80 + 10 = 90), and 91-99 continue counting the same way as the 70s did: quatre-vingt-onze (91), quatre-vingt-douze (92), up to quatre-vingt-dix-neuf (99). Once you reach 100, the system becomes clean and logical again: cent (100), cent un (101, no hyphen and no 'et' after cent), deux cents (200, with an -s since it's an exact multiple with nothing following), deux cent un (201, -s drops again once something follows), and so on up to mille (1000), which never takes an -s regardless of the multiplier: deux mille (2000), trois mille (3000).
Belgian and Swiss French actually use simpler, more base-ten-consistent words for 70 and 90 (septante, nonante — though huitante/octante for 80 is less universal), but standard European French as taught in most classrooms and as used in France uses the soixante-dix/quatre-vingts/quatre-vingt-dix system described above, so that's the priority to master.
Exemples
Ce sac coûte soixante-quinze euros.
This bag costs seventy-five euros.
75 = soixante ('sixty') + quinze ('fifteen').
Il y a quatre-vingts personnes dans le magasin.
There are eighty people in the store.
'Quatre-vingts' keeps the -s only when it's an exact multiple of 20 with nothing after it.
J'ai quatre-vingt-un ans.
I am eighty-one years old.
The -s on 'vingt' drops once a number follows; no 'et' before 'un' here (unlike soixante et onze).
Le prix total est de quatre-vingt-dix-neuf euros.
The total price is ninety-nine euros.
99 = quatre-vingt ('four-twenty'=80) + dix-neuf ('nineteen').
Elle a soixante et onze ans.
She is seventy-one years old.
71 uses 'et onze', the one exception with 'et' in the 70s range.
Ça fait cent euros, s'il vous plaît.
That'll be one hundred euros, please.
'Cent' alone, no article needed (not 'un cent').
Nous sommes deux cents personnes ici.
There are two hundred of us here.
'Cents' takes -s as an exact multiple of 100.
Le livre coûte deux cent cinquante euros.
The book costs two hundred fifty euros.
-s drops from 'cent' once a number follows it.
Cette voiture coûte vingt mille euros.
This car costs twenty thousand euros.
'Mille' is always invariable — never takes an -s.
J'ai économisé mille cinq cents euros.
I've saved one thousand five hundred euros.
Erreurs fréquentes
- Trying to say a single word for 70, 80, or 90 the way English does, instead of the compound forms soixante-dix / quatre-vingts / quatre-vingt-dix.
- Keeping the -s on 'quatre-vingts' when a number follows it (writing 'quatre-vingts-un' instead of the correct 'quatre-vingt-un', with the -s dropped).
- Adding 'et' before 'un' in the 80s the way it's used at 71 (soixante et onze), when in fact quatre-vingt-un has no 'et'.
- Adding an -s to 'mille' when it's multiplied (writing 'deux milles' instead of the correct 'deux mille', since mille is always invariable).
- Adding an article before 'cent' (saying 'un cent euros' instead of just 'cent euros').
Pourquoi ça trompe les anglophones
- English numbers 70-99 follow a completely regular base-ten pattern with dedicated words (seventy, eighty, ninety), so French's vigesimal (base-20) leftover system for this exact range has no equivalent logic to transfer from English.
- The -s on 'quatre-vingts' and 'cents' that appears only for exact multiples and disappears once a number follows is a spelling rule tied to a grammatical distinction (exact multiple vs. not) that English numbers don't mark at all.
- Doing arithmetic in your head while forming the number (80 = 4×20, so 'four-twenty'; 90 = 4×20+10) adds real-time cognitive load that native English number words never require.