How to Say "one-ninth" in Spanish
The Spanish word for “one-ninth” is “noveno” — B1 level.
English → SpanishB1
nounB1
mathematical fraction

Examples
Solo me queda un noveno del pastel.
I only have one-ninth of the cake left.
Un noveno de la población vive en la capital.
One-ninth of the population lives in the capital.
Divide el terreno en novenos.
Divide the land into ninths.
Using it as a math part
When you use it as a noun to mean a fraction, it is almost always masculine (el noveno).
Don't confuse with 'nueve'
Mistake: “Quiero un nueve del pastel.”
Correction: Quiero un noveno del pastel. Use 'nueve' for the number 9, but 'noveno' for the slice or piece.
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