Compound Interest Calculator
See how your money can grow over time with the power of compounding.
What is Compound Interest?
Compound interest is often referred to as "interest on interest." It is the process where the interest you earn on your investment is added back to the principal, forming a larger base on which future interest accumulates. Over long periods, this creates an exponential growth curve that can significantly increase your wealth compared to simple interest.
Albert Einstein famously called compound interest the "eighth wonder of the world," stating, "He who understands it, earns it; he who doesn't, pays it."
How This Calculator Works
This tool uses the standard compound interest formula with monthly contributions. Here is the breakdown of the inputs:
- Initial Investment: The lump sum of money you start with.
- Monthly Contribution: Money you add to the investment every month. Regular contributions are the key to building substantial wealth.
- Interest Rate: The annual rate of return you expect to earn. The S&P 500 has historically returned about 10% annually before inflation.
- Years to Grow: The duration of your investment. Time is the most powerful factor in compounding.
The Formula Used
The calculation assumes monthly compounding, which is standard for most savings accounts and investment projections. The math involves two parts: the future value of the lump sum and the future value of the series of monthly payments.
Formula: A = P(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) – 1) / (r/n)]
Strategies to Maximize Your Returns
To get the most out of compound interest, consider these three strategies:
- Start Early: Because compounding is exponential, the earlier years are worth less than the later years in dollar value, but they are crucial for setting the trajectory. Starting 5 years earlier can double your result.
- Increase Contributions: Even small increases in your monthly contribution ($50 or $100) can result in tens of thousands of dollars more over 20-30 years.
- Reinvest Dividends: Ensure that any earnings are automatically reinvested rather than withdrawn.
Frequently Asked Questions
What is a good interest rate to use?
For a conservative estimate on stock market investments (like index funds), 7% is often used as an inflation-adjusted average. High-yield savings accounts typically offer between 3% and 5%.
Does this account for inflation?
This calculator shows the nominal future value. To understand the purchasing power (real value), you might subtract the expected inflation rate (e.g., 3%) from your expected interest rate input.
Disclaimer: This calculator is for educational purposes only. It assumes a fixed rate of return, which does not reflect actual market volatility. Consult a financial advisor for personalized advice.