What is an index fund?
An index fund is a mutual fund or ETF that holds all (or a representative sample) of the stocks in a market index. The S&P 500 index fund, for example, holds the 500 largest US companies in the same proportions as the index itself. When you buy one share, you own a slice of all 500 companies.
The core advantage: automatic diversification, passive management, and ultra-low fees. An index fund tracking the S&P 500 might charge 0.03–0.05% per year, versus 1% or more for an actively managed fund that tries to "beat" the market.
Why index funds dominate FIRE
Most professional fund managers fail to beat the market consistently after fees. A 2022 S&P Dow Jones report found that 88% of large-cap fund managers underperformed the S&P 500 over 15 years. By holding an index fund, you guarantee yourself market-level returns minus minimal fees — which beats most professionals.
For FIRE specifically, this matters enormously. A 0.5% annual fee difference on a $500,000 portfolio is $2,500 per year. Over 20 years, that compounds to nearly $100,000 in lost wealth. Index funds eliminate that leak.
The three core index funds for a simple portfolio
- Total US Market Index (VTI, VTSAX, FSKAX) — all ~3,500 US-listed stocks. Includes large, mid, and small caps.
- Total International Index (VXUS, VTIAX, FTIHX) — all major developed and emerging markets outside the US.
- Bond Index (BND, VBTLX, FXNAX) — US government and investment-grade corporate bonds. Lowers volatility.
A simple three-fund portfolio (60% US, 30% international, 10% bonds) requires only three trades and rebalances once per year. This is the foundation of most FIRE plans.
ETFs vs mutual funds
Index funds come in two wrapper types: ETFs (exchange-traded funds) and mutual funds. For FIRE purposes, it does not matter much. ETFs trade intraday like stocks; mutual funds settle at day-end. Both offer low fees. Choose whichever your brokerage makes easiest.
Where to buy index funds
Most brokerages offer low-cost index funds: Vanguard (VTI, VTSAX), Fidelity (FSKAX, FTIHX), and Schwab (SWTSX). Avoid high-fee platforms or robo-advisors that layer fees on top of index funds. Pick a brokerage, open an account, and invest directly.
Getting started
- Open a brokerage account (Vanguard, Fidelity, or Schwab).
- Decide your target allocation (e.g., 70% US, 20% international, 10% bonds).
- Buy index funds in those proportions — your first purchase is the hardest, second-guessing yourself.
- Set up automatic monthly contributions if possible.
- Rebalance annually if the allocation drifts more than 5%.
That is it. You do not need to pick stocks, time the market, or monitor daily. Index funds make investing simple enough for anyone.