Compound Interest Calculator

Compound interest formula

Compound interest means interest earns more interest over time. With no extra deposits, the standard formula is A = P(1 + r/n)^nt. A is the ending balance, P is the starting principal, r is the annual rate as a decimal, n is the number of compounding periods per year, and t is the number of years. A 6% annual rate entered in the widget becomes 0.06 in the formula. Monthly compounding uses n = 12, quarterly compounding uses n = 4, and annual compounding uses n = 1.

Monthly contributions make the calculation more practical for saving plans. This calculator converts the nominal annual rate into an effective monthly rate and then steps through the plan month by month. Each month grows the existing balance and then adds the contribution. That approach keeps the result easy to understand and lets fractional terms, different compounding frequencies and regular deposits work together without any server or spreadsheet.

Reading the breakdown

Final balance is the estimated account value at the end of the term. Total contributions are the principal plus all monthly deposits. Interest earned is final balance minus contributions. If the interest figure is negative or unexpectedly small, check for a very short term, a zero rate, or a contribution amount that was typed into the wrong field. The formula line updates live so you can see how many months were modeled and what effective monthly rate was used.

Planning notes and privacy

Use compound interest estimates for scenario planning, not as a promise from a bank or broker. Real accounts can include variable returns, fees, taxes, withdrawal rules, contribution timing, inflation and market risk. Try conservative and optimistic rates to see a range of possible outcomes. All values stay in your browser; the page does not upload balances, fetch rates or store a financial profile.