Simple Interest

What Is Simple Interest?

Simple interest is interest calculated only on the original principal, the initial amount deposited or borrowed. Unlike compound interest, it does not add previously earned or owed interest back into the balance before computing the next round, so the interest amount each period stays tied to the same starting figure. If a fixed principal earns simple interest at a set annual rate, the interest added each year is the same dollar amount, and the balance grows in a straight line rather than curving upward. This makes simple interest easier to calculate and predict. It appears in some short-term loans and certain fixed arrangements, and it serves as a useful contrast that clarifies exactly what compounding adds when interest is instead figured on a growing balance.

Why It Matters

Distinguishing simple from compound interest helps explain why two accounts advertising the same rate can end with different balances. Under simple interest, the interest generated in early periods does not go on to earn further interest, so the total accumulates more slowly than it would under compounding at the same rate. Over short periods the gap is small, but as the time horizon lengthens the difference widens, because compounding lets interest build on interest while simple interest does not. Knowing which method applies is part of reading a savings product or loan accurately, since the annual percentage yield captures compounding while a plain nominal rate resembles the simple-interest view. The contrast makes the value of time and compounding frequency concrete rather than abstract.