Simple Interest Calculator
Last updated July 2, 2026
Last reviewed July 8, 2026
Simple interest calculates interest only on the original principal, not on previously earned interest. The formula is principal multiplied by rate multiplied by time. A $10,000 balance at 5 percent simple interest for three years earns $1,500, ending at $11,500. That makes simple interest easy to understand and useful for short-term notes, informal loans, and quick comparisons where compounding is not part of the agreement.
The key distinction is that simple interest does not snowball. Compound interest adds interest to the balance and then earns interest on that larger amount in later periods. Over short horizons the difference can be small; over long horizons it becomes large. That is why a simple-interest calculator should not be used to estimate long-term investment growth unless the product truly pays simple interest.
Use the simple interest calculator when the agreement or assumption explicitly uses simple interest. Enter the principal, annual rate, and time period, then compare the result with a compound-interest calculation if the product or loan compounds interest in practice. The right formula depends on the contract, not on which result looks more convenient.