Compound Interest: The Most Powerful Force in Finance
Understand how your money can grow exponentially over time and why starting early matters more than investing more.
What Is Compound Interest?
Compound interest is the interest you earn on both your original deposit and the interest that has already been added. Unlike simple interest—which only grows based on your principal—compound interest accelerates because each period's gains become part of the next period's base. Over decades, this difference becomes enormous.
The Compound Interest Formula
The standard formula is A = P(1 + r/n)^(nt), where P is the principal, r is the annual interest rate, n is the number of compounding periods per year, and t is the number of years. When you add regular contributions the math gets more complex, which is exactly why a calculator is helpful.
Why Starting Early Matters
If you invest $200/month starting at age 25 with a 7% average annual return, you'll have roughly $525,000 by age 65. Wait until 35 to start and you'll only have about $244,000—less than half—despite investing for just 10 fewer years. The first dollars you invest have the longest time to compound and therefore do the most work.
How Compounding Frequency Affects Growth
The more frequently interest compounds—daily vs. monthly vs. annually—the faster your balance grows. A 5% annual rate compounded daily yields slightly more than the same rate compounded annually. For most long-term investing, the difference between monthly and daily compounding is small, but between annual and monthly it can be noticeable.
The Rule of 72
A quick mental shortcut: divide 72 by your annual interest rate to estimate how many years it takes to double your money. At 6%, your investment doubles roughly every 12 years. At 8%, every 9 years. This simple rule helps you gauge long-term growth without a calculator.
Put This Into Practice
Apply what you've learned using the WealthWise tool built for this exact purpose.
Try the Investment CalculatorFrequently Asked Questions
What is compound interest in simple terms?
Compound interest means you earn interest on your interest. Each time interest is calculated, it's added to your balance, and the next calculation uses that larger balance.
How much does $10,000 grow with compound interest?
$10,000 at 7% compounded annually grows to about $19,672 in 10 years, $38,697 in 20 years, and $76,123 in 30 years—without any additional contributions.
Is compound interest good or bad?
It depends which side you're on. Compound interest works in your favor when you're saving or investing. It works against you when you carry debt—credit card balances compound rapidly because unpaid interest gets added to your balance.