Hola Simone

Great question! In Spanish, both "cien por ciento" and "ciento por ciento" can be used, but they are used in different contexts.

"Cien" is used directly before a noun, meaning "one hundred". For example, "cien personas" (one hundred people). Therefore, "cien por ciento" (one hundred percent) and "cien por cien" (one hundred per hundred) are grammatically correct and commonly used to express 100%.

However, "ciento" is used in numbers greater than one hundred, such as "ciento uno" (one hundred and one) or "ciento cincuenta" (one hundred and fifty). When it comes to percentages, "ciento por ciento" is an idiomatic expression in Spanish that is widely understood and accepted, even though it might seem redundant from a grammatical perspective.

So, while "cien por ciento" and "cien por cien" are correct and precise, "ciento por ciento" has become a standard expression in Spanish to emphasize completeness or entirety, often used for emphasis or rhetorical effect.

Apologies for the late reply, and I hope this clarifies your doubt!

Un saludo

Silvia