Simple Interest Calculator

Most long-term lending today, mortgages, credit cards, standard investment accounts, uses compound interest instead, which is exactly why simple interest can feel almost unfamiliar if you haven't run into it. It still shows up specifically in short-term personal loans, certain auto financing structures, and promissory notes between individuals, contexts where the simplicity and predictability of flat-rate interest actually works in everyone's favor.

The formula itself, I = P × r × t, is refreshingly direct compared to a compound interest calculation, which is also exactly why it's a staple of introductory finance coursework, there's no iterative math involved, just a straightforward multiplication.

Enter your principal, the annual interest rate, and the duration in years. The calculator outputs total interest earned or owed, along with your final payout or repayment balance.

For a loan or deposit lasting less than a full year, convert the timeframe into a decimal, six months becomes 0.5 years, for instance, which plugs directly into the same formula without needing a separate calculation method. Your figures stay entirely private, calculated locally.