How Compound Interest Works
Albert Einstein famously called compound interest the "eighth wonder of the world," and for a good reason. Unlike simple interest, which only pays interest on your original deposit, compound interest pays interest on your interest.
Over long periods, this creates a snowball effect, allowing your wealth to grow exponentially rather than linearly. Our Compound Interest Calculator helps you visualize this growth by factoring in your starting principal, expected rate of return, and ongoing monthly contributions.
Key Terms You Need to Know
To maximize your investments, it is crucial to understand the variables driving your returns:
Starting Principal: The initial amount of money you invest or deposit into the account.
Monthly Contribution: Additional money you deposit on a regular basis. Consistency here is key—even small monthly additions can double or triple your final balance over several decades.
Interest Rate (APR): The annual rate of return you expect to earn. For stock market investments, an average historical return is often estimated between 7% and 10%. For high-yield savings accounts, this matches the bank's stated rate.
Time Horizon: The number of years your money will remain invested. Because compound interest is exponential, time is the most powerful factor in the equation.
The Power of Compounding Frequencies
The Compounding Frequency dictates how often the interest you earn is added to your principal balance. The more frequently interest is compounded, the faster your money grows.
Annually: Interest is calculated and added once a year.
Monthly: Interest is calculated and added 12 times a year. (Most common for savings accounts and mortgages).
Daily / Continuously: Interest is added every single day, maximizing the compounding effect.
Example: If you invest $10,000 at a 5% interest rate for 10 years, compounding annually yields an end balance of $16,288.95. Compounding daily pushes that balance to $16,486.65.
APR vs. APY: What is the Difference?
When dealing with bank accounts and investments, you will often see two different percentages: APR (Annual Percentage Rate) and APY (Annual Percentage Yield).
APR is the base, stated interest rate for a whole year, without taking compound interest into account.
APY is the effective rate you actually earn in a year, because it includes the effect of compounding.
Banks typically advertise APY for savings accounts because it represents a higher, more attractive number. Conversely, they advertise APR for loans and mortgages because it represents a lower number to borrowers. Our calculator automatically handles the complex conversions between APR, APY, and your chosen compounding frequency to give you an exact future value.
The Rule of 72
Want to do a quick mental calculation? Use the Rule of 72 to estimate how long it will take to double your money. Simply divide the number 72 by your expected annual interest rate.
If you expect an 8% return: 72 ÷ 8 = 9 years. It will take approximately 9 years to double your initial investment.