Quick answer
uno · veintiuno · cien · ciento uno
Regular from 31 upwards. The traps are un before a masculine noun, cien versus ciento, and the gender of hundreds.
The building blocks
| Number | Spanish |
|---|---|
| 1–10 | uno, dos, tres, cuatro, cinco, seis, siete, ocho, nueve, diez |
| 11–15 | once, doce, trece, catorce, quince |
| 16–19 | dieciséis, diecisiete, dieciocho, diecinueve |
| 20–29 | veinte, veintiuno, veintidós … veintinueve |
| 30+ | treinta y uno, cuarenta y dos, cincuenta y tres … |
| tens | treinta, cuarenta, cincuenta, sesenta, setenta, ochenta, noventa |
The teens and twenties are written as one word. From treinta on, you write three words with y in the middle.
uno loses its o
Before a masculine noun, uno becomes un. Before a feminine noun it becomes una. This applies to every compound ending in one.
Hundreds and thousands
Exactly 100 is cien. Anything above it uses ciento, with no y after it.
Mil never takes un in front and never becomes plural when counting: dos mil, tres mil.
Memory trick
Say the number out to yourself: if it ends in "one" and a noun follows, it has to shrink to un or una.
Typical mistakes
Tengo uno hermano.
Tengo un hermano.
Uno stands alone. In front of a noun it drops the o.
Ciento euros.
Cien euros.
Exactly one hundred is cien. Ciento only appears in 101–199.
Ciento y veinte.
Ciento veinte.
The y goes between tens and units only, never after the hundreds.
Check yourself
Hay ___ mujeres en la sala. (200)