Key Takeaway
Index funds and ETFs both track the same indexes and have nearly identical long-term returns. For most beginners in a Roth IRA or 401k, a traditional index mutual fund is slightly simpler. In a taxable brokerage account, ETFs are more tax-efficient.
If you have started researching investing, you have almost certainly run into both terms: index funds and ETFs. People often use them interchangeably, which causes genuine confusion. They are similar in many ways but meaningfully different in others. Here is a clear breakdown.
What They Have in Common
Both index funds and ETFs are designed to track an index, which is a list of stocks following a specific set of rules. The most common example is the S&P 500, which tracks the 500 largest US companies by market capitalization. Both types of funds give you instant diversification across dozens or hundreds of companies with a single purchase. Both have very low expense ratios compared to actively managed funds. And both have historically outperformed the majority of actively managed funds over long time horizons.
How Index Funds Work
A traditional index mutual fund is bought and sold directly through a brokerage at the end-of-day price, called the Net Asset Value (NAV). You submit an order during the day, and the transaction settles at the closing price. There is no bid-ask spread. Many index funds have a minimum initial investment, though Fidelity and Schwab have eliminated minimums on their most popular funds.
How ETFs Work
An ETF (Exchange-Traded Fund) trades on a stock exchange throughout the day, just like a share of Apple or Amazon. You can buy or sell at any moment the market is open, at the current market price. ETFs also have a bid-ask spread, which is the difference between the price you can buy at and the price you can sell at. For large, liquid ETFs like VTI or SPY, this spread is typically a penny or less and is not meaningfully expensive.
The difference between index funds and ETFs is mostly a technicality for long-term investors. Both will make you wealthy over time if you buy and hold them. Do not let the comparison paralyze you from starting. - JL Collins, author of The Simple Path to Wealth
Key Practical Differences
- Tax efficiency: ETFs are generally more tax-efficient in taxable brokerage accounts because of how they handle redemptions. This matters in a taxable account but is irrelevant inside a Roth IRA or 401k where you are not taxed on gains anyway.
- Minimum investment: You can buy a single share of an ETF (VTI is roughly $280 per share). Index mutual funds sometimes have minimums, though the major brokerages have largely eliminated these.
- Automatic investing: Index mutual funds are easier to automate. You can set up a recurring investment of a specific dollar amount. With ETFs, you typically have to purchase whole shares, though many brokerages now offer fractional share purchasing.
- Intraday trading: ETFs can be bought and sold during market hours. For long-term investors, this is mostly irrelevant and can actually be a negative by tempting people to trade more often.
Simple Decision Rule
Inside a Roth IRA or 401k: use whichever is available at your brokerage with the lowest expense ratio. At Fidelity that is FSKAX (0% expense ratio). At Vanguard that is VTSAX or VTI. In a taxable brokerage account: slightly favor ETFs for tax efficiency.
The Expense Ratio Is What Actually Matters
The most important number for both index funds and ETFs is the expense ratio, which is the annual fee charged as a percentage of your investment. The best index funds and ETFs charge between 0% and 0.05% per year. An actively managed fund might charge 1% or more. On a $100,000 portfolio, the difference between a 0.03% expense ratio and a 1% expense ratio is $970 per year, every year, before considering the compounding effect of that cost over decades.
The fund type, index fund versus ETF, matters far less than the expense ratio and your consistency in contributing over time. Pick either one with a low expense ratio and keep buying it regularly.