Compound Interest Calculator

Compound interest is interest earned on both your original money and the interest it has already earned. Because each period’s earnings get added back to the balance, they start earning too — so growth accelerates over time rather than staying flat.

For a lump sum: A = P(1 + r/n)^(nt), where P is the principal, r is the annual rate, n is how many times a year it compounds, and t is the number of years. When you also add regular contributions, each deposit is grown to the end date and summed (an annuity), which is what this calculator does for you.

The more often interest compounds — daily vs. monthly vs. annually — the sooner earnings start earning, so the balance grows slightly faster. The effect is real but usually small compared to your rate of return, contribution amount, and time invested.

Early on, your contributions do most of the work. Over long horizons the balance tips: compound interest can eventually contribute more than everything you paid in. Time is the biggest lever — the earlier you start, the more of your final balance comes from growth rather than deposits.

It depends on where your money sits. A high-yield savings account might be a few percent; the S&P 500 has averaged roughly 10% a year before inflation over the long run, though real returns swing widely year to year. Use a conservative figure and remember this is an estimate, not a guarantee.

No. It’s an educational estimate that assumes a constant annual return, which real markets never deliver. Actual results vary and are not guaranteed. For decisions about your own money, consider your full situation or talk to a professional.