Compound Interest — The Eighth Wonder
Compound interest is the process by which investment returns themselves generate additional returns over time. The future value formula is: FV = PV × (1 + r)^n, where PV is the present value (initial investment), r is the annual return rate, and n is the number of years. At a 10% annual return, $10,000 grows to: $10,000 × (1.10)^10 = $25,937 in 10 years; $67,275 in 20 years; $174,494 in 30 years; $452,593 in 40 years. Note the acceleration: the last ten years (30→40 years) add $278,099 to the portfolio — more than the first thirty years combined. This exponential acceleration is the compounding effect. The Rule of 72 provides a quick mental shortcut: divide 72 by the annual return rate to estimate how many years it takes for money to double. At 10% annual return: 72/10 = 7.2 years to double. At 7%: 72/7 ≈ 10 years to double. Einstein reportedly called compound interest the eighth wonder of the world — whether or not he said it, the mathematics justify the hyperbole. The critical implication: time in the market is the single most powerful variable in personal wealth accumulation. Beginning at age 22 vs. age 32 is not a 10-year disadvantage; at 10% annual return, a 22-year-old who invests $500/month for ten years (age 22–32) then stops will have more at age 62 than a 32-year-old who invests $500/month continuously for thirty years (age 32–62) — the early investor's head start is never overcome.