be decided; be certain toDescribes something that has been settled or decided, or—especially in the pattern ~に決まっている—that something is bound to happen. In 腹が決まっている, it means that one's mind or resolve is made up.
JA: 来月から新しい仕事を始めることが決まっている。
EN: It has been decided that I will start a new job next month.