How Compound Interest Works

Quick Answer

Compound interest is interest earned on both your original money and the interest it has already earned. Because each period grows a slightly larger balance, the total builds faster and faster the longer you leave it. This is why time is the most important ingredient in saving and investing.

To project growth you need four things: the starting amount, the interest rate, how it compounds, and how many years. Adding a regular monthly contribution accelerates the result and frequently contributes more to the final total than the starting sum.

How It Works

Each compounding period, interest is added to the balance. The next period then earns interest on that larger balance, including the interest already credited. Therefore, over many periods this snowball effect produces growth that a simple, non compounding calculation cannot match.

Two levers matter most: the rate and the time. A higher rate compounds faster, and more years give the snowball longer to roll. This is the time value of money in action: a dollar invested today is worth more than a dollar later because it has longer to compound. That principle drives most long term investment growth.

How Does Compounding Frequency Change the Result?

Compounding frequency is how often interest is added to the balance: yearly, quarterly, monthly, or daily. More frequent compounding adds interest sooner, so the next period earns on a slightly larger balance. However, the effect is real but modest. For example, at 7% on $10,000 for 20 years, yearly compounding reaches about $38,697, while monthly compounding reaches about $40,387. The rate and the number of years move the result far more than the frequency does, which is why more frequent compounding helps less than people expect.

How to Include Monthly Contributions

Most people add money over time rather than investing a single lump sum. Enter a monthly contribution and each deposit compounds for the months remaining. Over long horizons these regular contributions add more to the final balance than the starting amount. This is the engine behind retirement and education savings growth: a steady monthly habit, left to compound for decades, does the heavy lifting.

Examples

A lump sum left to grow. $10,000 invested at 7% compounded monthly for 20 years grows to about $40,387. You contribute nothing extra; the growth is entirely compounding on the original amount.

Adding monthly contributions. Start with $1,000, add $200 a month, and assume 8% for 30 years. The balance grows to roughly $309,000, of which only $1,000 was the starting sum and $72,000 was contributed. The rest is compounding at work.

The Formula

The future value of a principal is:

FV = P (1 + r/n)^(n·t)

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. Regular contributions add a second term, the future value of an annuity, which this tool computes for you.

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