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Numbers

The basic, “cardinal” numbers of Esperanto are:

unu — one ok — eight
du — two naŭ — nine
tri — three dek — ten
kvar — four cent — hundred
kvin — five mil — thousand
ses — six miliono — million (106)
sep — seven miliardo — billion (109)

These words are all roots, and are used without further modification before nouns to convey quantity:

tri saĝuloj three wisemen
sep nanoj seven dwarves
ok etaj boacoj eight tiny reindeer
naŭ muzoj nine muses

One can combine the roots to produce numbers greater than ten, writing such combinations as separate words by powers of ten:

Ĉi tiuj iras ĝis dek unu. These go to eleven.
kvardek tagoj, kvardek noktoj forty days, forty nights
La Frenezaj Okdek Ok The Crazy Eighty-Eight
Du mil unu: Kosma odiseado Two Thousand One: A Space Odyssey*
Vi devos pagi al mi ... MILIONON DA DOLAROJ. Pardonu ... CENT MILIARDOJN DA DOLAROJ! You’re going to have to pay me ... ONE MILLION DOLLARS. Sorry ... ONE HUNDRED BILLION DOLLARS!
naŭcent naŭdek naŭ mil naŭcent naŭdek naŭ boteloj da biero ĉe la muro nine hundred ninety-nine thousand nine hundred ninety-nine bottles of beer on the wall
*Mil, miliono, and upward are written as separate words, so ducent (“two hundred”), but du mil (“two thousand”).

For numbers greater than a billion, one can add -iliono (a million to the power of x) or -iliardo (a thousand times a million to the power of x) to the numbers one through ten:

duiliono trillion (1012)
duiliardo quadrillion (1015)
triiliono quintillion (1018)
triiliardo sextillion (1021)
dekiliono novemdecillion (1060)
dekiliardo vigintillion (1063)

Numbers can be made into nouns denoting groups or sets by adding -o. In these cases, numbers greater than ten are written as one word:

ĵaza trio a jazz trio
du dekduoj da ovoj two dozen eggs

Ordinal Numbers

Ordinal numbers are those that express a thing’s position in a series, such as first, second, third. In Esperanto, ordinals are formed by adding -a or -e to the equivalent cardinal number:

Unue vi devas trovi ... ankoraŭ arbustaron! Firstly you must find ... another shrubbery!
La tria premio estas maldungo. Third prize is you’re fired.
Nu, la hodiaŭo estis tago speciala por mi. Ĝi estis la cent sepdek naŭa tago sukcedanta en kiu mi faris precize la saman aferon! Well, today was a special one for me. It was the hundred and seventy-ninth day in a row where I did exactly the same thing!

When asking for something requiring an ordinal number, one uses kioma, which means “which one of the series?”:

“Kiu tago estas?” “Estas Kristonasko, sinjoro!” “What day is it?” “It’s Christmas Day, sir!”
“Kioma tago estas?” “Estas la 25a, sinjoro!” “What day is it?” “It’s the 25th, sir!”

Fractional Numbers

Fractional numbers are those that express a value that is not a whole number, eg. half, a fourth, etc. In English, as in many European languages, these are mostly indistinguishible from ordinal numbers (eg., the fifth Beatle vs. a fifth of the Beatles), but in Esperanto are marked by the suffix -on-:

dekduono a twelfth part
dek kaj duono ten and a half
dek duonoj ten halves
kvardek tri centonoj forty-three hundredths
kvardek tricentonoj forty three-hundredths
Trionon de la tero voris pudeloj. A third of the earth was devoured by poodles.
Duonon el vi mi ne konas duone tiom, kiom mi volus; kaj mi ŝatas malpli ol duonon el vi duone tiom, kiom vi meritas. I don’t know half of you half as well as I should like, and I like less than half of you half as well as you deserve.

Multiplicative Numbers

Multiplicative numbers are those like English double, triple, and quadruple. In Esperanto, they are formed from the cardinal numbers by adding -obl-:

Duobligu vian plezuron, duobligu vian amuzon! Double your pleasure, double your fun!
Vivasekuro saldas trioble kiam oni mortas dum negocvojaĝo. Life insurance pays off triple if you die on a business trip.
Kvaroble kvin estas dudek. Four times five is 20.

Distributive Numbers

Distributive numbers are formed from cardinal numbers by adding -op-, which means “x at a time”:

La sturmantoj venis duope. The stormtroopers came as a pair, or two at a time.
La sablanoj defilas unuope por kaŝi sian nombron. The sandpeople march single file to hide their numbers.
Kiomope vi vendas la biletojn? How many tickets at a time can you sell?

Arithmetic

Some common operations in arithmetic:

Dek kvin plus tri estas dek ok. Fifteen plus three equals eighteen.
Dek kvin minus tri estas dek du. Fifteen minus three equals twelve.
Dek kvin multiplikite per tri estas kvardek kvin. Fifteen times three equals forty-five.
Dek kvin dividita per tri estas kvin. Fifteen divided by three equals five.
Dek je la dua potenco estas cent. Ten to the power of two is a hundred.

Time

Time in Esperanto is expressed in terms of which hour it is (first, second, third, etc.) and the minutes or fraction of an hour before or after the hour in question:

Kioma horo? Kiomas? What hour is it?
Estas la unua. It’s one o’clock.
Estas la dekunua kaj duono. It’s eleven-thirty.
Estas kvin antaŭ la sesa. It’s five minutes to six.
Estas la kvina kaj kvindek kvin. It’s five fifty-five.