Compound Interest Calculator

Compound interest is the process where the interest you earn on an investment is added back to the principal, so the next interest calculation is based on a larger amount. Unlike simple interest — which is calculated only on the original sum — compound interest grows on itself, snowballing over time. The longer your money stays invested and the more frequently interest compounds, the larger the snowball becomes. This is why financial planners call compounding the eighth wonder of the world: small, consistent contributions made early in life can outgrow much larger lump sums invested later, simply because they have decades to compound.

Each compounding period, your balance is multiplied by (1 + rate ÷ periods). With monthly compounding at 7% annual, the monthly factor is roughly 1.00583, applied twelve times per year. After year one your $10,000 becomes about $10,723; year two starts compounding from that new base, not the original $10,000. Add a recurring contribution and each new dollar joins the growth engine immediately. Three levers control the outcome: how much you start with, how much you add regularly, and how long you let it run. Time, more than rate of return, usually does the heaviest lifting.

Imagine you invest $5,000 today and add $200 every month into a low-cost index fund averaging 7% annually. After 10 years you have roughly $42,000 — about $13,000 of which is pure interest. Stretch that to 30 years and the same plan grows to roughly $275,000, with more than $193,000 coming from compounded returns rather than your contributions. If you waited 10 years to start, you'd end up with only about $122,000 — less than half. The lesson: starting early with a modest amount almost always beats starting late with a larger one. Use the calculator above to model your own scenario.