What is Compound Interest? Why Did Einstein Call It the "Eighth Wonder"?
Compound interest is interest calculated on the initial principal, which also includes all the accumulated interest from previous periods. In short: "interest on interest."
Unlike simple interest, compound interest generates earnings from previous earnings, creating exponential growth. The longer the time, the more dramatic the effect.
Compound Interest Formula
Lump sum future value:
FV = PV × (1 + r)^n
Annuity future value (regular contributions):
FV = PMT × [(1 + r)^n - 1] ÷ r
Simple vs. Compound: What's the Difference?
Invest $100,000 at 8% annual return for 30 years:
Simple Interest
$340K
Fixed $8,000/year interest
Total interest: $240K
Compound Interest
$1.01M
Interest earns interest
3x more than simple
The Rule of 72
A quick mental math shortcut:
Years to double ≈ 72 ÷ Annual Return (%)
- 6% return → doubles in ~12 years
- 8% return → doubles in ~9 years
- 12% return → doubles in ~6 years
How Ordinary People Build Wealth with Compounding
- Start early: Investing $500/month at 8% from age 25 = ~$1.5M by 60. Starting at 35 = ~$650K. Those 10 years cost you $850K.
- Stay consistent: Don't stop during market downturns. Lower prices mean more shares for the same money.
- Minimize fees: Fund management fees compound against you. Choose low-cost index funds (ETFs).
- Don't interrupt: Compounding's worst enemy is withdrawing early. Keep investment accounts separate.
- Be realistic: 8-10% is a reasonable long-term expectation for stock index funds.
Common Investment Returns
| Investment |
Long-term Return |
Risk |
Best For |
| Savings Account | 0.5% | Minimal | Emergency fund |
| Money Market | 4-5% | Low | Short-term savings |
| Bond Index | 4-6% | Low-Med | Conservative investors |
| Stock Index (S&P 500) | 10% | Medium | Long-term growth |
| Individual Stocks | Variable | High | Experienced investors |
Frequently Asked Questions
Compound interest vs. dollar-cost averaging: which matters more?
They work together. Early on, contributions matter more. Later, compounding dominates. Start contributing early and let compounding take over.
Is 8% return guaranteed?
No. 8-10% is a long-term (10+ year) historical average. Short-term returns vary significantly. The key is time in the market, not timing the market.
I'm 40. Is it too late to start?
Absolutely not. You still have 20+ years until retirement. The best time to start was yesterday. The second best time is today.