No rule for this one
No ending or word group predicts the article of Saft. Learn it as a pair and say it out loud: der Saft.
der Saft
It's der Saft. Saft is masculine, plural die Säfte.
Ein Orangensaft, bitte.
An orange juice, please.
No ending or word group predicts the article of Saft. Learn it as a pair and say it out loud: der Saft.