What Is Compound Interest?
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. In simple terms, it's 'interest on interest' — and it's the reason your money can grow exponentially over time.
While simple interest only grows linearly (the same dollar amount each year), compound interest accelerates because each period's interest becomes part of the base for future calculations. This snowball effect is what makes long-term investing so powerful.
The Math Behind Compound Interest
The compound interest formula is A = P(1 + r/n)^(nt), where A is the future value, P is the principal, r is the annual interest rate, n is the number of times interest compounds per year, and t is the time in years.
For example, $10,000 at 7% compounded monthly for 30 years becomes $81,165 — over 8 times the original investment. The same amount with simple interest would only reach $31,000. That's the power of compounding.
The Rule of 72
A quick way to estimate how long it takes to double your money is the Rule of 72. Simply divide 72 by your annual interest rate. At 8%, your money doubles in approximately 9 years (72 ÷ 8 = 9). At 12%, it doubles in about 6 years.
This rule works reasonably well for rates between 4% and 20%, and it's a great mental math tool for quickly evaluating investment opportunities.
Why Starting Early Matters
Consider two investors: Alice starts investing $200/month at age 25 and stops at 35 (10 years, $24,000 total). Bob starts at 35 and invests $200/month until 65 (30 years, $72,000 total). At 8% annual return, Alice ends up with more money at 65 than Bob — despite investing only a third as much.
This counterintuitive result demonstrates why financial advisors emphasize starting early. The extra years of compounding more than compensate for the smaller total contribution.
Compound Interest and Debt
The same force that grows your investments can work against you with debt. Credit card interest compounds, meaning unpaid interest adds to your balance and generates more interest. At 22% APR, a $5,000 balance with minimum payments can take over 20 years to repay.
Understanding this dual nature of compound interest — friend for savings, foe for debt — is fundamental to financial literacy.
Questions people ask
- Is compound interest the same as compound returns?
- They're related but not identical. Compound interest is a specific calculation on a fixed rate. Compound returns in investing include capital gains, dividends, and reinvestment — which also benefit from the compounding effect but with variable returns.
- How often should interest compound for maximum benefit?
- More frequent compounding yields slightly higher returns. Daily compounding earns more than monthly, which earns more than annually. However, the difference between daily and monthly is quite small.