Compound Interest Calculator

Compound interest means you earn interest on both your original deposit and on the interest you have already earned. Each compounding period, the balance grows by the period rate, so growth accelerates over time. Monthly contributions in this calculator are added at the end of each month and start earning interest the following month.

For the lump sum part: future value = P x (1 + r/f)^(f x t), where P is the principal, r is the annual rate as a decimal, f is the number of compounding periods per year, and t is the number of years. Each monthly contribution grows from the month it is added, so the future value of all contributions is M x ((g^n - 1) / (g - 1)), where M is the monthly contribution, n is the total number of months, and g is the effective monthly growth factor (1 + r/f)^(f/12). This page does that math for you automatically.

More frequent compounding means interest is added to your balance sooner, so it starts earning its own interest earlier. For the same nominal annual rate, daily compounding always beats monthly, which beats quarterly, which beats semi-annually, which beats annually - though the difference between daily and monthly is usually small.

Contributions are new money that immediately starts compounding. Over long periods, regular contributions often grow to be larger than the original principal. The chart above shows this: the gold area (your contributions) usually dominates early, while the violet area (interest earned) catches up and can overtake it in later years.

With a 0% rate there is no growth: your ending balance equals your principal plus every monthly contribution added together, and the interest earned is exactly $0. The calculator handles this edge case - no division by zero, the contributions simply add up.

No. This is a projection for education only, not financial advice. Real investments carry risk, returns are not guaranteed, and taxes, fees, and inflation are not included in these numbers.

No. Every calculation happens in your browser; nothing you type is sent to a server or stored.