Compound Interest: The Simple Math That Creates Most Wealth
Published May 10, 2026
The Math That Changes Everything
Compound interest is interest earned on interest. That sounds simple. The implications are not.
If you invest $10,000 at 10% annual return:
- After 1 year: $11,000
- After 2 years: $12,100 (interest on the original + interest on last year's interest)
- After 10 years: $25,937
- After 30 years: $174,494
You turned $10,000 into $174,000 without adding another dollar. That's compounding.
The Rule of 72
The Rule of 72 is the fastest way to estimate how long it takes to double your money: divide 72 by the annual return rate.
At 8% return: 72 ÷ 8 = 9 years to double At 10% return: 72 ÷ 10 = 7.2 years to double At 12% return: 72 ÷ 12 = 6 years to double
This also works for debt — at 24% credit card interest, your debt doubles every 3 years.
Why Starting Early Matters More Than Amount
Two investors:
- Investor A: Invests $5,000/year from age 25-35 (10 years), then stops. Total invested: $50,000.
- Investor B: Invests $5,000/year from age 35-65 (30 years). Total invested: $150,000.
At a 7% average return, who has more at age 65?
Investor A: ~$602,000 Investor B: ~$567,000
The person who invested 1/3 as much money ends up with more — because they started 10 years earlier and gave compounding more time to work.
The Three Levers
Compounding magnitude is controlled by three things:
- Rate of return: Even 1-2% difference compounds dramatically over decades
- Time: The most powerful lever. Start earlier, not larger.
- Consistency: Stopping and starting kills compounding. Consistency beats amounts.
The Inflation Warning
Compounding works against you too. At 3% inflation, $100 today is worth $55 in 20 years. Keeping money in a savings account earning less than inflation means you're losing purchasing power every year — slowly, invisibly, inexorably.
The Takeaway
You don't need a high income to build wealth. You need time and consistency. The person who starts investing $200/month at 25 will likely retire wealthier than the person who starts investing $2,000/month at 45.
The best time to start was yesterday. The second best time is today.
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