Both ETFs and traditional index mutual funds can track the same underlying index — say, the S&P 500 — and produce very similar long-run returns. The differences that matter are structural, not about performance. Here’s a concrete comparison.
The Example: Two Ways to Track the S&P 500
- An S&P 500 ETF (a fund that trades on an exchange like a stock, buyable in single shares throughout the trading day)
- An S&P 500 index mutual fund (bought and sold once per day at the closing price, often through a retirement account)
Both aim to replicate the same index, and their long-run performance historically tracks within a fraction of a percentage point of each other, largely explained by differences in expense ratio.
Cost Comparison
Expense ratios for major S&P 500 ETFs commonly run around 0.03%-0.09% annually. Comparable index mutual funds are often similar, though some older or actively-marketed mutual fund share classes still charge 0.5% or more. On a $10,000 investment held for 20 years at a 7% average return:
- At a 0.03% expense ratio: ending balance ≈ $38,500
- At a 0.50% expense ratio: ending balance ≈ $35,400
That roughly $3,100 gap over 20 years comes entirely from the fee difference, not performance — a reminder to check the expense ratio regardless of which structure (ETF or mutual fund) you choose.
Practical Differences
Minimum investment: ETFs can typically be bought for the price of a single share (often $50-$500 depending on the fund), while some mutual funds require a minimum initial investment of $1,000-$3,000.
Trading flexibility: ETFs trade throughout the day like a stock, with a price that fluctuates in real time. Mutual funds are priced once, after market close. For long-term investors, this difference rarely matters in practice.
Tax efficiency in a taxable account: ETFs generally generate fewer taxable capital gains distributions than mutual funds, due to differences in how shares are created and redeemed. This mostly matters in a regular brokerage account — it’s largely irrelevant inside a tax-advantaged account like a 401(k) or IRA.
Which One to Actually Choose
For most people investing through a workplace 401(k), the choice is often made for you — 401(k) plans typically only offer mutual fund options. For an individual brokerage account or IRA, ETFs are usually the simpler and slightly cheaper default, provided the expense ratio is comparably low. The single factor worth checking before either purchase is the expense ratio — a fund tracking the same index at 10x the cost of a competitor is losing money for no additional benefit.