What compound interest really means
Compound interest is the reason small, consistent savings can turn into large sums over time. Unlike simple interest — which only ever pays you a return on your original deposit — compound interest pays you a return on your deposit and on all the interest you have already earned. Each period, your balance is a little bigger, so the next interest payment is a little bigger too. Over years and decades this "interest on interest" effect snowballs, which is why Albert Einstein reportedly called it the eighth wonder of the world.
How to use this compound interest calculator
Enter four things: your starting principal, the annual interest rate, the number of years you will stay invested, and how often the interest compounds (annually, monthly, daily, and so on). The calculator returns your final balance, the total interest earned, and the overall percentage growth. Try changing the compounding frequency from annual to monthly with the same rate — you will see the final figure rise, because more frequent compounding gives interest more chances to earn interest.
The formula
The calculator uses A = P(1 + r/n)nt, where P is the 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. For example, $10,000 at 5% compounded monthly for 10 years grows to about $16,470 — roughly $6,470 of that is interest you never deposited.
Getting the most from compounding
Time is the most powerful lever, far more than the rate itself: starting five years earlier often beats chasing a slightly higher return. Regular contributions accelerate the effect dramatically, and leaving the interest untouched (rather than withdrawing it) is what keeps the snowball rolling. The same maths works against you with credit-card debt, where the balance compounds in the lender's favour.