Simple vs. Compound Interest
Simple interest earns returns only on the original principal. Compound interest earns returns on both the principal and previously earned returns — so each period's gains become the base for the next.
In investing, compounding works when you reinvest dividends (DRIP) and avoid selling. Every withdrawal or sale resets the clock on that portion of capital.
The Numbers That Show Why Time Matters
Scenario: $10,000 at 7% annually for 30 years (pre-tax, simplified)
Simple interest: $700/year × 30 = $21,000 in interest → ~$31,000 total.
Compound interest: ~$76,000 total.
The $45,000 difference comes purely from reinvestment over time — not skill or luck.
The last 10 years typically generate more absolute dollars than the first 20 combined. This is why starting early — even with a small amount — matters more than the size of the initial investment.
The Rule of 72
A quick mental math shortcut: 72 ÷ annual return % ≈ years to double your money.
The rule also shows why costs matter: shaving 1% off your annual return adds roughly 1–2 years to your doubling time.
What Kills Compounding
- Frequent trading — commissions, spreads, taxes, and poor timing all reduce net returns.
- High expense ratios — 1% vs. 0.03% annually looks small; over 30 years it can be tens of thousands of dollars.
- Panic selling in downturns — locking in losses and missing the recovery resets the compounding clock.
- Leveraged ETF decay — daily rebalancing erodes value in volatile markets even when the index is flat.
Historical Context: S&P 500
The S&P 500 has compounded at roughly 10% annually (nominal, pre-tax) over the long run — through two world wars, multiple recessions, the dot-com crash, the financial crisis, and COVID. Investors who stayed invested captured those returns; those who sold during panics did not.
Compounding requires two inputs: time in the market, and the psychological discipline to stay invested through 30–50% drawdowns. The math is straightforward; the behavior is the hard part.
Early vs. Middle vs. Late Stages
Early stage: Absolute dollar gains feel small. DCA contributions add more value than investment returns at this point. Focus on consistency, not returns.
Middle stage: Portfolio value grows large enough that annual fluctuations feel painful. Pre-set rules for downturns become essential here.
Late stage: Sequence-of-returns risk emerges. Withdrawals, tax efficiency, and asset allocation shifts matter more than maximizing growth.
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