How to Say "one hundred" in Spanish
The most common Spanish word for “one hundred” is “cien” — use 'cien' when referring to the exact number 100, especially when it stands alone or modifies a noun directly..
cien
/syen/ or /thyen//ˈsjen/

Examples
Necesitamos cien sillas para el evento.
We need one hundred chairs for the event.
Mi abuela cumplió cien años la semana pasada.
My grandmother turned one hundred years old last week.
El costo de la reparación es cien dólares.
The cost of the repair is one hundred dollars.
The Cien/Ciento Rule
Use 'cien' ONLY when the number 100 stands alone or comes directly before a noun. If it is part of a larger number (like 101, 150) or used as a standalone noun/number (like a percentage), you must use 'ciento' instead.
Always Singular
'Cien' always stays the same, regardless of whether the noun it refers to is masculine or feminine (e.g., 'cien días' and 'cien noches').
Mixing Cien and Ciento
Mistake: “Compró ciento libros.”
Correction: Compró cien libros. (Use 'cien' when it is right next to the noun 'libros'.)
Using Cien in Large Numbers
Mistake: “Doscientos cien (200 + 100).”
Correction: Trescientos. (If it's part of a compound number, 'cien' changes to 'ciento', and then compounds into 300, 400, etc.)
ciento
/syén-toh//ˈθjento/

Examples
Tengo ciento cincuenta dólares en mi cuenta.
I have one hundred fifty dollars in my account.
El aumento fue del cien por ciento.
The increase was one hundred percent.
Si sumas cien y ciento uno, tienes doscientos uno.
If you add one hundred and one hundred one, you have two hundred one.
The 'Cien' Rule (Apocope)
The full form 'ciento' is only used when the number 100 is immediately followed by another number (e.g., 'ciento uno'). Otherwise, it shortens to 'cien' when standing alone or before a noun (e.g., 'cien personas').
Use with Percentages
When talking about percentages, you must use the shortened form 'cien': 'cien por ciento' (one hundred percent).
Using 'Ciento' before nouns
Mistake: “Compré ciento libros.”
Correction: Compré cien libros. (You must drop the 'to' when it comes right before the thing you are counting.)
Confusing 'Cien' and 'Ciento'
Mistake: “El número es cien tres.”
Correction: El número es ciento tres. (Keep the 'to' when the number continues.)
'Cien' vs. 'Ciento' Confusion
Related Translations
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