The simplest way interest can work
Simple interest calculates a return purely on the original principal amount, for the entire duration — unlike compound interest, where earned interest itself starts earning interest. The formula is about as straightforward as finance math gets:
Interest = Principal × Rate × Time
Because the principal used in the calculation never changes, interest accrues at a constant amount each period rather than accelerating — a straight line rather than a curve when plotted over time.
Where you'll encounter it
Simple interest shows up in some short-term loans, certain bonds, and a handful of promotional savings products. For anything longer-term — mortgages, typical retirement accounts, most savings products — compound interest is the standard, and the Compound Interest Calculator on this site is the more relevant tool for those scenarios.