The big picture
Spanish numbers are built in blocks, left to right. The asymmetry that trips up learners: 21–29 are written as one fused word (veintidós), but 31–99 use a separated y connector (treinta y dos). And the false friend that wrecks finance: billón = 10¹², not 10⁹.
Zero to twenty
0–15 are unique words. 16–19 are written as one fused word (dieciséis, not diez y seis), with an accent mark on dieciséis.
21 – 99: fused, then y
The 20s stay fused, just like the teens — veintidós, veintitrés, veintiséis with accent marks. From 30 onward, the connector y appears as a separate word: treinta y uno, cuarenta y cinco, noventa y nueve.
21 – 29: one fused word, with accents
30 – 99: decade + y + unit
Hundreds: cien vs ciento
Cien is exactly 100, or used before a noun / larger unit: cien libros, cien mil, cien millones. Ciento takes over for 101–199. And the hundreds 200–900 agree in gender, with three irregular forms you can't predict.
In the wild
Thousands
Mil = 1,000 — and never un mil. It's invariant for exact counts: dos mil, treinta mil, cien mil. The plural miles exists, but only as a noun meaning "thousands of" (miles de personas).
Millions & billones
Millón and billón are nouns, not number-words — they pluralize, take determiners, and need de before a following noun. And the long-scale system means billón = 10¹², the opposite of English "billion."
Long-scale, same as French milliard, German Milliarde, Italian miliardo. Spanish has no single word for 10⁹ — it's literally "a thousand millions." And un billón is 1.000.000.000.000.
Short scale. The cognate billón in a Spanish text means a thousand times more than English "billion." Wire transfer disasters waiting to happen.
Things to remember
Five rules that will save you from the most common mistakes.
Reading is one thing.
Hearing it at speed is another.
The companion iOS app generates random numbers in your chosen range and reads them aloud in Spanish. Five minutes a day.
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