📈Compound Interest Calculator

Compound interest is interest on your interest. In the first period your money earns a return on the amount you put in. In the next period it earns a return on that original amount plus the interest already credited, and so on. Because each round of growth builds on a slightly larger base, the balance does not climb in a straight line; it curves upward, accelerating the longer it is left alone.

Time is the most powerful input in the whole calculation. The earliest dollars you invest have the most years to compound, so they end up contributing far more than their face value. This is why starting sooner, even with a small amount, usually beats starting later with a larger one: you are buying years of compounding that cannot be added back afterward.

A single deposit grows on its own, but adding money every month builds the balance from two directions at once. Each new contribution increases the principal, and the existing principal keeps earning returns. Over time the two effects reinforce each other, which is why steady monthly investing tends to outperform a one-time lump sum of the same total amount spread across the same period.

The table below shows future values for different monthly amounts at a 7% annual return, starting from a zero balance. Notice how the gap between the columns widens the longer the money compounds, and how tripling the monthly amount roughly triples every result because contributions scale proportionally.

Small changes in the assumed rate produce large changes in the final balance, and the effect grows with the time horizon. A percentage point or two may look minor on paper, but compounded over decades it can shift the outcome by a meaningful margin. For that reason it is worth running the calculator with a few different rates rather than relying on a single optimistic figure.

Time amplifies everything. As a concrete example, a $1,000 starting balance plus $100 a month at 7% grows to $19,318 after 10 years. Of that total, $13,000 is money you actually contributed and $6,318 is interest. Extend the horizon and the interest portion keeps climbing as a share of the balance, because the earlier contributions have had longer to compound.

This calculator assumes a single, fixed annual rate that stays the same every year. Real investment returns do not behave that way: they vary from year to year, can be negative in some periods, and are affected by fees, taxes, and inflation that this model does not account for. Treat the output as an illustration of how compounding works, not a forecast of what any specific investment will deliver.

Use the results to compare scenarios and understand the mechanics of long-term saving, not as a guarantee. This tool is for education and general planning only and is not investment advice. For decisions about your own money, consider consulting a qualified financial professional who can account for your full situation.

FV is the future value, the projected balance at the end of the term.

P is the principal, your starting balance, and PMT is the amount you add each month.

r is the monthly rate, equal to the annual rate divided by 12, and n is the total number of months.

The first term grows your starting balance; the second term sums the growth of every monthly contribution.