Numbers

Why English numbers are easy to skip past

English numbers are mostly invariant — three is three whether it modifies cat or cats, he or she. The only irregularity most learners notice is one vs. a/an, and ordinals (first, second, third) that don’t follow the -th pattern.

Spanish numbers carry more grammar: some of them agree in gender with the noun they modify, and the way tens and units combine changes shape depending on the range. Once you see the pattern, though, it’s almost entirely predictable — this doc covers the small set of rules that generate every number you’ll need.

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The core rule: 0–29 are memorized, 30+ is a formula

One fact explains most of what follows in this doc: numbers 0 through 29 are individual words you memorize, but from 30 onward every number is built from a small, fixed set of pieces (tens word + y + units word).

• dieciséis through veintinueve (16–29) are single fused words, not phrases
• treinta (30) and up use the pattern [tens] y [units]: treinta y uno, cuarenta y dos
• cien/ciento, hundreds, and thousands each add one more small rule on top

Keep this in mind — every “why does this one look different?” question below traces back to one of these small, learnable rules, not an exception to memorize individually.

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0–15 — pure memorization

#	Spanish	#	Spanish	#	Spanish
0	cero	6	seis	12	doce
1	uno	7	siete	13	trece
2	dos	8	ocho	14	catorce
3	tres	9	nueve	15	quince
4	cuatro	10	diez	—	—
5	cinco	11	once	—	—

There’s no shortcut here — these are the atoms every larger number is built from.

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16–29 — fused “ten + unit” words

From 16 to 29, Spanish fuses diez y X and veinte y X into single words, with small spelling adjustments (an accent appears where the fused stress would otherwise be lost).

#	Spanish	Literally	#	Spanish	Literally
16	dieciséis	diez y seis	23	veintitrés	veinte y tres
17	diecisiete	diez y siete	24	veinticuatro	veinte y cuatro
18	dieciocho	diez y ocho	25	veinticinco	veinte y cinco
19	diecinueve	diez y nueve	26	veintiséis	veinte y seis
20	veinte	(its own word)	27	veintisiete	veinte y siete
21	veintiuno	veinte y uno	28	veintiocho	veinte y ocho
22	veintidós	veinte y dos	29	veintinueve	veinte y nueve

You may see the old three-word spelling (diez y seis, veinte y uno) in older texts — it’s outdated but still readable. Modern standard Spanish always fuses these into one word.

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30–99 — tens word + y + unit, as separate words

Unlike 16–29, from 30 upward the tens and units stay separate words, joined by y.

Tens	Spanish	Tens	Spanish
30	treinta	70	setenta
40	cuarenta	80	ochenta
50	cincuenta	90	noventa
60	sesenta	—	—

#	Spanish	#	Spanish
31	treinta y uno	65	sesenta y cinco
42	cuarenta y dos	78	setenta y ocho
53	cincuenta y tres	89	ochenta y nueve
47	cuarenta y siete	99	noventa y nueve

The y only ever connects a tens word to a units word (treinta y uno). It never connects hundreds to tens (ciento uno, not ciento y uno) — see below.

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100+ — cien vs. ciento, and hundreds that agree in gender

Value	Spanish	Rule
100 (exactly, or before a noun)	cien	cien libros, cien personas
101–199	ciento + rest	ciento uno, ciento cincuenta — no y after ciento
200	doscientos / doscientas	agrees with the noun’s gender
300	trescientos / trescientas	agrees with the noun’s gender
400	cuatrocientos / cuatrocientas	agrees
500	quinientos / quinientas	irregular stem
600	seiscientos / seiscientas	agrees
700	setecientos / setecientas	irregular stem
800	ochocientos / ochocientas	agrees
900	novecientos / novecientas	irregular stem

#	Word	Meaning
1	cien casas	100 houses
2	ciento cinco	105
3	doscientos libros	200 books (masculine)
4	doscientas casas	200 houses (feminine)
5	quinientos euros	500 euros
6	mil	1,000 — invariant, no gender agreement

cien is used for exactly 100 and right before a noun (cien años — “100 years”); ciento is used when more follows (ciento veinte — “120”). The hundreds words (200–900) are the only numbers besides uno that change form for gender — see below.

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uno — shortens to un before a masculine noun

uno behaves like the indefinite article: it drops its final -o to un before a masculine singular noun, and becomes una before a feminine one. This carries through every compound number ending in uno (21, 31, 41…).

#	Spanish	Meaning
1	un libro	one/a book
2	una casa	one/a house
3	veintiún años	21 years
4	veintiuna semanas	21 weeks
5	treinta y un días	31 days
6	treinta y una personas	31 people
7	cuarenta y un euros	41 euros
8	Tengo uno.	I have one. (no noun follows — full form stays)

Note the written accent: veintiún (not veintiun) — the fused 21–29 forms need it to keep the stress on the last syllable when -o is dropped.

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1,000 and beyond

Value	Spanish	Notes
1,000	mil	never un mil — just mil
2,000	dos mil	mil doesn’t pluralize in counting numbers
100,000	cien mil	—
1,000,000	un millón	takes un, and needs de before a noun
2,000,000	dos millones	pluralizes, unlike mil

#	Spanish	Meaning
1	mil novecientos noventa y nueve	1999
2	dos mil veinticinco	2025
3	un millón de personas	one million people
4	tres millones de euros	three million euros

millón needs de before the noun it counts (un millón de habitantes), because grammatically millón is a noun, not a number word like mil. Contrast with mil personas — no de.

Castellano-specific formatting: Spain uses . as the thousands separator and , as the decimal separator — the reverse of English. 1.999,50 in Spanish is 1,999.50 in English.

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Ordinal numbers — 1st through 10th

Ordinals agree in gender and number like adjectives, and are far less common past “tenth” — Spanish switches to cardinal numbers for higher ordinals in everyday speech (el piso once, not el undécimo piso, for “the eleventh floor”).

#	Masculine	Feminine	Meaning
1st	primero	primera	first
2nd	segundo	segunda	second
3rd	tercero	tercera	third
4th	cuarto	cuarta	fourth
5th	quinto	quinta	fifth
6th	sexto	sexta	sixth
7th	séptimo	séptima	seventh
8th	octavo	octava	eighth
9th	noveno	novena	ninth
10th	décimo	décima	tenth

primero and tercero shorten before a masculine singular noun

Just like uno → un, primero and tercero drop their final -o before a masculine singular noun (not before feminine, and not before plural).

#	Spanish	Meaning
1	el primer día	the first day
2	la primera vez	the first time
3	el tercer piso	the third floor
4	los primeros días	the first days (plural — no shortening)
5	la tercera casa	the third house

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Common mistakes

• ❌ un mil → ✅ just mil — Spanish never puts uno before mil
• ❌ ciento y cinco → ✅ ciento cinco — no y directly after ciento
• ❌ un millón personas → ✅ un millón de personas — millón needs de before a noun
• ❌ veinte y uno as the modern spelling → ✅ veintiuno — fused, one word, for 16–29
• ❌ Forgetting gender agreement on hundreds → ✅ doscientas personas, not doscientos personas, when the noun is feminine
• ❌ uno libro → ✅ un libro — uno shortens to un before a masculine singular noun

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Summary

• 0–15 are memorized individually; 16–29 fuse into single words (dieciséis, veintiuno); 30+ join tens and units with y as two words
• cien before a noun or exactly 100; ciento when more follows, with no y after it
• Hundreds (200–900) agree in gender with the noun: doscientos libros / doscientas casas
• uno → un and primero/tercero → primer/tercer before a masculine singular noun
• mil never takes un and doesn’t pluralize when counting; millón does pluralize and needs de before a noun
• Ordinals agree in gender and number like adjectives, but Spanish prefers cardinals past “tenth” in everyday speech

See also: Articles — the next topic once you can read, pronounce, and count in Spanish.