How to Buy Your First Index Fund
ETF or mutual fund? Roth IRA or taxable? Here is how to actually buy your first index fund and set it on autopilot.
Don't have time? Here's what you need to know:
- 1ETF and mutual fund versions of the same index fund hold identical stocks — pick whichever is easier to automate
- 2Max your 401(k) match first, then Roth IRA, then taxable account
- 3Automatic monthly investing removes emotion and builds wealth through consistency
- 4The expense ratio difference between ETF and mutual fund versions is typically 0.01% — both are fine
Index Fund ETFs vs Index Fund Mutual Funds
Index funds come in two wrappers: ETFs and mutual funds. Same underlying stocks, different packaging. The Vanguard 500 Index Fund (VFIAX, mutual fund) and VOO (ETF) hold identical stocks. The ETF version trades throughout the day like a stock. The mutual fund version trades once per day after market close at the net asset value (NAV).
For most beginners, the practical differences are small. ETFs have slightly lower expense ratios in some cases (VOO is 0.03% vs VFIAX at 0.04%). Mutual funds allow automatic investing in exact dollar amounts at Vanguard without fractional share support. At Fidelity and Schwab, fractional ETF shares make this advantage moot. Pick whichever format your broker makes easier to automate.
| Feature | Index Fund ETF (e.g., VOO) | Index Fund Mutual Fund (e.g., VFIAX) |
|---|---|---|
| Trades | Anytime during market hours | Once per day after close |
| Minimum investment | $1 (fractional shares) | $3,000 at Vanguard, $0 at Fidelity |
| Expense ratio | 0.03% | 0.04% |
| Automatic investing | Broker-dependent | Always available |
| Tax efficiency | Slightly better | Slightly worse |
Where to Hold Your Index Fund
If you have a 401(k) with employer matching, contribute enough to get the full match first — that is free money. Next, max out a Roth IRA ($7,000 per year in 2024) if your income qualifies. Roth IRA gains grow tax-free forever. After that, use a regular taxable brokerage account.
Inside your 401(k), pick the lowest-cost S&P 500 or total market index fund available. Many plans offer institutional share classes with expense ratios under 0.05%. For your Roth IRA and taxable accounts, VTI or VOO are the standard choices.
Tip: At Vanguard, the mutual fund version (VFIAX) is easier to automate. At Fidelity, the ETF version (VOO) with fractional shares is just as easy and slightly cheaper.
Set It and Forget It: Automatic Index Fund Investing
The entire point of index fund investing is that it should take almost no ongoing effort. Set up automatic contributions on a monthly schedule aligned with your paycheck. Choose the same ETF or mutual fund every time. Turn on dividend reinvestment. Then check your account once a quarter — not once a day.
This approach — called dollar-cost averaging — means you buy more shares when prices are low and fewer when prices are high. Over 20-30 years, the exact day you invest each month barely matters. What matters is that you invested every single month.
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Frequently Asked Questions
Should I buy the ETF or mutual fund version of an index fund?
At Fidelity or Schwab with fractional shares, the ETF version is slightly cheaper and equally easy to automate. At Vanguard, the mutual fund version is easier to set up for automatic investing since Vanguard does not offer fractional ETF shares. Performance is nearly identical either way.
What if my 401(k) does not have a good index fund?
Look for the fund with the lowest expense ratio that tracks a broad market index. If the cheapest option is 0.20% or higher, contribute enough to get the employer match, then direct additional savings to a Roth IRA where you have full control over fund selection.
Can I buy an index fund with just $50?
Yes. At Fidelity, you can buy $50 worth of VOO using fractional shares. At Vanguard, their index mutual funds have a $3,000 minimum for most share classes, but their target-date funds start at $1,000. If you have less than $1,000, Fidelity is the better starting point.
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Alex Harrington
CFA Level II Candidate, Finance & Economics
Alex Harrington is an independent ETF researcher and personal finance writer with over 8 years of experience analyzing exchange-traded funds. A CFA Level II candidate with a background in economics, Alex has reviewed 800+ ETFs and helped thousands of beginners build their first investment portfolios through clear, jargon-free education.
This content is for educational purposes only and does not constitute financial advice. Past performance does not guarantee future results. Consult a licensed financial advisor before making investment decisions.