Simple Interest Calculator

Simple interest is the most straightforward way to work out how much a loan, deposit, or investment will earn (or cost) over time, without compounding getting in the way. This calculator takes a principal amount, an annual interest rate, and a length of time in years, and instantly shows how much interest accrues and what the final total comes to.

The formula behind it is the classic I = P × r × t: interest equals the principal multiplied by the annual rate (as a decimal) multiplied by the time in years. A $1,000 principal at a 5% annual rate over 3 years earns $1,000 × 0.05 × 3 = $150 in interest, for a final amount of $1,150. Unlike compound interest, simple interest is always calculated on the original principal alone: the interest earned in one period never itself starts earning interest in the next, which is what makes the math a single multiplication instead of an exponential formula.

This is the calculation behind many everyday scenarios: a short-term personal loan, a simple promissory note, a certificate of deposit that pays simple rather than compound interest, or just estimating how a fixed-rate savings amount grows over a known stretch of time. It's also a common way to sanity-check numbers on a loan document or a finance homework problem, since the formula is simple enough to verify by hand once you have the three inputs.

Every figure updates live as you type, with no "Calculate" button to press, and the time field accepts fractional years (like 0.5 for six months or 2.5 for two and a half years) for exact partial-year periods. Every calculation runs as plain JavaScript directly in your browser, so nothing you type is ever sent to a server, logged, or stored anywhere.