The most common way to say "I'm tired" in Spanish is:

• Estoy cansado (for a male speaker)
• Estoy cansada (for a female speaker)

This phrase uses the verb estar (to be) in the first person singular form "estoy" plus the adjective "cansado/a," which means tired. It is widely understood and used in both formal and informal contexts across Spanish- speaking countries.

Other ways to express tiredness include expressions like "Estoy agotado/a" (I'm exhausted), "Tengo sueño" (I'm sleepy), or colloquial forms such as "Estoy hecho polvo" (I'm worn out) mainly used in Spain.

So, the simplest and most direct translation is "Estoy cansado" if the speaker is male or "Estoy cansada" if the speaker is female.