Simple Interest Calculator

Simple interest is calculated only on the original principal amount during the whole period. This means interest is not added to the balance to generate new interest. For example, if you lend $1,000 at 10% per year for 2 years with simple interest, the interest is always calculated on the same $1,000.

Fill in the initial amount, interest rate and time. Then choose whether the time is in months or years and click "Calculate simple interest". The tool will show the total interest and final amount including interest. You can adjust the values as many times as you like to simulate different scenarios.

The calculation follows the classic simple interest formula: I = P × r × t, where I is the interest, P is the principal, r is the interest rate per period in decimal form (for example, 5% = 0.05) and t is the time in years. If you choose the time in months, the calculator converts it to years by dividing the number of months by 12. The final amount is obtained by adding the interest to the principal: Final amount = P + I.

With simple interest, the interest is always calculated on the original principal — it never grows on top of itself. With compound interest, the interest generated in each period is added to the balance, so future interest is calculated on a larger amount. For short-term loans or deposits at low rates, the difference is small. Over long periods, compound interest can produce significantly larger totals.

Simple interest is commonly used in short-term personal loans, car financing, some savings accounts, installment plans, and student loans in certain countries. It is also the standard for calculating interest on treasury bills and other short-term government securities. Because the math is straightforward, it is often preferred for educational purposes and for contracts where transparency is important.