Compound Interest: How Your Money Grows Over Time

Albert Einstein allegedly called compound interest the "eighth wonder of the world." Whether he said it or not, the math is undeniable: compound interest is the single most powerful force in personal finance.

Simple vs Compound Interest

• Simple interest — You earn interest only on the principal. $10,000 at 7% for 30 years = $10,000 + $21,000 = $31,000
• Compound interest — You earn interest on the principal AND the accumulated interest. $10,000 at 7% for 30 years = $76,123

The difference: $45,123 — more than 4× your original investment, just from compounding.

The Compound Interest Formula

Simple vs Compound Growth — Visualized

Simple interest grows in a straight line (you earn on the principal only). Compound interest bends into a curve — every year the past interest starts earning too.

How Compounding Frequency Matters

$100,000 at 7% for 30 years:

The Rule of 72

Divide 72 by your interest rate to estimate how many years it takes to double your money:

• At 6%: 72 ÷ 6 = 12 years to double
• At 8%: 72 ÷ 8 = 9 years to double
• At 10%: 72 ÷ 10 = 7.2 years to double

Inflation: The Silent Killer

A 7% return with 3% inflation means your real return is only ~4%. After 30 years at 7% nominal, $100,000 grows to $761,230 — but in today's dollars, that's only about $314,000 of purchasing power.

The Bottom Line

1. Start early — time is more important than the amount invested
2. Compound interest turns small, consistent contributions into large sums
3. Always account for inflation when planning long-term investments
4. Use the Rule of 72 to quickly estimate doubling time

Disclaimer: This guide is for informational purposes only and does not constitute financial advice. Past performance does not guarantee future results.