How do you say “A piece of cake” in French?

Both English and French curiously associate effortless tasks with cake. In French, "c'est du gâteau" is the direct colloquial match for "it's a piece of cake," routinely used to reassure someone that a challenge won't present any real difficulty. Notice the syntactic difference: French drops the idea of a physical "piece" or "slice" and uses the partitive construction ("du gâteau") rather than a countable noun.

While "c'est du gâteau" is understood by everyone, native speakers in informal contexts are equally fond of the idiom "les doigts dans le nez" (literally "fingers in the nose"), which implies doing something so easily you could do it while picking your nose. If you need a more neutral or professional tone, "un jeu d'enfant" ("child's play") is the standard alternative.