What is Compound Interest?
Compound interest is often called the eighth wonder of the world — a phrase attributed to Albert Einstein. Unlike simple interest, which is calculated only on your initial principal, compound interest is calculated on the principal plus all previously accumulated interest. This creates an exponential growth curve that accelerates over time.
The longer your money compounds, the more dramatic the effect. The difference between 10 years and 30 years of compounding is not 3×, it is often 8–10×. This is why starting to invest early is one of the most powerful financial decisions you can make.
The Rule of 72
A quick mental math trick: divide 72 by your annual interest rate to estimate how many years it takes to double your money.
- At 4% — money doubles in ~18 years
- At 7% — money doubles in ~10.3 years
- At 10% — money doubles in ~7.2 years
- At 12% — money doubles in ~6 years
Why Start Early?
Consider two investors: Alice invests $10,000 at age 25 and never adds another dollar. Bob waits until age 35 and invests $10,000. Both earn 7% annually. By age 65, Alice has $149,745. Bob has $76,123. Alice ends up with nearly twice as much — just by starting 10 years earlier, not by investing more.
Time is the most powerful variable in compound interest. The earlier you start, the more of that exponential curve you capture.
Frequently Asked Questions
What is compound interest?
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest, it grows exponentially over time — often called the "eighth wonder of the world."
What is the difference between compound and simple interest?
Simple interest is calculated only on the principal. Compound interest is calculated on the principal plus all previously earned interest. Over long periods, the difference becomes enormous.
How does compounding frequency affect returns?
More frequent compounding (daily vs. annually) results in slightly higher returns. For example, $10,000 at 7% compounded annually for 20 years = $38,697. Compounded monthly = $40,064. The difference grows larger with higher rates and longer periods.
What is the Rule of 72?
The Rule of 72 is a quick way to estimate how long it takes to double your money. Divide 72 by your annual interest rate. At 7%, your money doubles in about 72 ÷ 7 = 10.3 years.
Why do monthly contributions matter so much?
Regular contributions compound over time just like the principal. Adding $200/month to a $10,000 investment at 7% for 20 years results in $148,000+ instead of $38,000. Consistent contributions are often more impactful than the initial amount.
What is APY (Annual Percentage Yield)?
APY (or Effective Annual Rate) is the real annual return accounting for compounding frequency. A 7% nominal rate compounded monthly gives an APY of 7.229%. APY is what you actually earn and is always higher than the nominal rate when compounding is more frequent than annual.