Simple Interest Calculator

Interest in its simplest form accrues on the original principal alone, never on the earnings it produces. This tool models that behavior and sets it side by side with compound interest so the gap between the two becomes obvious. That gap matters: as the 2026 rate environment takes shape, even a fraction of a percentage point can change long-term outcomes, which is why anyone weighing a loan, a short-term investment, or a first lesson in finance benefits from seeing both figures together.

The arithmetic follows I = P * R * T. Here 'I' is the interest earned, 'P' is the principal amount, 'R' is the annual interest rate written as a decimal, and 'T' is the time period in years. Compound interest works differently: it applies the rate to the principal plus whatever interest has already piled up in earlier periods, so each cycle earns interest on previously earned interest.

Because simple interest ignores that snowball effect, it tends to understate how money actually grows over a long horizon. Two input habits keep the math honest: state the rate as an annual figure and express time in years. One more factor shapes the real result. Inflation, projected at around 2.5% for 2026, quietly eats into the purchasing power of whatever simple interest you collect.