Latin Numbers

Type "Latin numbers" into a search box and you will get two different things tangled together: the number words (unus, duo, tres) and the Roman numerals (I, II, III). They are not the same thing, and sorting them out is half the point of this page. Here are the cardinals and ordinals as words, the Roman numerals as symbols, and the one grammar fact that actually matters - that only the first three numbers decline.

Cardinals: one to twenty

Two things to notice. The lower teens are decem plus a digit, squeezed together: tredecim is three-ten, quindecim five-ten. But 18 and 19 count backwards from twenty: duodeviginti is two-from-twenty and undeviginti is one-from-twenty. It looks odd, but the Romans thought of 18 as "20 minus 2", and the same subtraction returns at 28 and 29, 38 and 39, all the way up.

The tens, hundreds and thousands

Compound numbers read much as English does. Twenty-one is viginti unus (or unus et viginti, one-and-twenty, both are correct), forty-seven is quadraginta septem, and a hundred and twenty-three is centum viginti tres. Two thousand is duo milia, and the plural milia takes a genitive of what it counts: duo milia hominum, two thousand men, literally two thousands of men.

The one grammar fact: only 1, 2 and 3 decline

This is the whole grammatical burden of Latin numbers, and it is refreshingly small. Only the first three numbers change their endings. Everything from quattuor (4) up to centum (100) is indeclinable - it sits unchanged whatever case the sentence needs.

• unus declines like an adjective, agreeing in case, number and gender: unus (m), una (f), unum (n).
• duo has its own pattern: duo (m and n), duae (f), with duos / duas in the accusative.
• tres is tres for masculine and feminine, tria for neuter.

So tres libri is three books and tria verba is three words, the number quietly agreeing with what it counts. From quattuor upward you can stop worrying: quattuor libri, quattuor verba, quattuor rerum - the number never moves. The hundreds from ducenti (200) up do decline, like ordinary plural adjectives, because they behave as quantities of things.

Ordinals: first, second, third

Ordinals are ordinary adjectives, so they agree in case, number and gender like any other:

These are the source of a great many English words - primary, secondary, quintessence, decimal - and of names still in use: Henricus Octavus is Henry the Eighth, and secundus is where second comes from. Because they are adjectives, they behave exactly like the ones in our example sentences: prima hora, the first hour; tertio die, on the third day.

Latin numbers are not Roman numerals

Here is the confusion this page exists to clear up. The number word and the number symbol are two different systems, and the Romans used both, just as we write "7" and say "seven".

The numerals are built from seven symbols - I, V, X, L, C, D, M - with smaller-before-larger meaning subtraction (IV is 4, IX is 9). The words are the language. If you came here to read a clock face, an inscription or a copyright date, you want the numerals; if you came to read or speak Latin, you want the words. Most people who search "Latin numbers" quietly want one specific one of these, and now you can tell which.

Keep going

Numbers are a lesson-sized corner of the language, and one of the friendlier ones. The free Latin course teaches them in context, the Latin example sentences page shows them at work in real sentences, and How to learn Latin sets out the frequency-first method that gets you reading. If you are weighing Latin against its neighbours, Latin vs Greek and Latin vs Italian put the numbers in company.