Index Funds vs. ETFs: Which Should You Choose in 2025?

Index funds and ETFs are the two most popular ways to invest in diversified, low-cost portfolios. They both track market indices, charge minimal fees, and outperform most actively managed funds over time. But they are not identical. Here is a data-driven comparison to help you choose.

The Market Size

The shift toward passive investing has been one of the defining trends in finance:

Combined index fund and ETF assets: $19.26 trillion (December 2024, ICI)

ETF total net assets: Crossed $10 trillion for the first time in 2024

Net share issuance of ETFs: Crossed $1 trillion for the first time in 2024

Index funds' share of long-term fund assets: 48% (up from 19% in 2010)

The growth has been dramatic. In just 14 years, index investing went from a fifth of the market to nearly half. This is not a fad — it is a structural shift in how Americans invest.

Cost Comparison

Both index funds and ETFs are cheap, but ETFs have a slight edge:

Average index ETF expense ratio: 0.15% (Morningstar, 2024)

Average index mutual fund expense ratio: 0.05-0.10% for the lowest-cost providers (Vanguard, Fidelity, Schwab)

Fidelity ZERO funds: 0.00% expense ratio (FNILX, FZROX, FZIPX, FZILX) — literally free

At the lowest cost tier, the difference between a 0.03% ETF and a 0.00% index fund is negligible. On a $100,000 portfolio, that is $30 versus $0 per year. The practical cost difference between major index funds and ETFs is now essentially zero.

Tax Efficiency: ETFs Win

This is where ETFs have a meaningful structural advantage. In 2024:

Roughly 5% of ETFs distributed capital gains

Roughly 40% of mutual funds distributed capital gains

Average ETF capital gains distribution: More than a percentage point less than the average mutual fund distribution

The reason is the ETF creation/redemption mechanism. ETFs use "in-kind" transactions — authorized participants exchange baskets of securities for ETF shares rather than cash. Under Section 852(b)(6) of the tax code, these in-kind transfers are tax-exempt, allowing ETFs to offload appreciated securities without triggering capital gains for shareholders.

Mutual funds, by contrast, must sell securities for cash when investors redeem shares. If those securities have appreciated, the capital gains are distributed to all remaining shareholders — even those who did not sell.

For taxable brokerage accounts, this makes ETFs significantly more tax-efficient. In tax-advantaged accounts (401k, IRA), the difference does not matter since gains are not taxed annually. Use our Tax Calculator to see how capital gains distributions affect your tax bill.

Trading and Flexibility

ETFs:

Trade throughout the day like stocks at real-time prices

Can be bought and sold instantly during market hours

Support limit orders, stop-loss orders, and options

No minimum investment beyond the price of one share (many brokers offer fractional shares)

Index Mutual Funds:

Trade once per day at the closing net asset value (NAV)

Orders placed during the day execute at the 4:00 PM closing price

No intraday trading capabilities

Some funds have minimum investments ($1,000-$3,000), though Fidelity and Schwab have $0 minimums

For long-term investors who buy and hold, the trading difference is irrelevant. You are not day-trading your retirement portfolio. But if you value flexibility and real-time pricing, ETFs offer more control.

Automatic Investing

This is where index mutual funds have an advantage. Most brokerages allow automatic recurring investments into mutual funds — for example, $500 on the 1st of every month, automatically. This makes dollar-cost averaging effortless.

ETF automatic investing has improved but is still less seamless. Some brokerages (Fidelity, Schwab) now offer automatic ETF purchases, but the experience varies by platform.

If your investment strategy relies on automated monthly contributions (which it should for most people), index mutual funds provide the smoothest experience.

Which to Choose: A Decision Framework

Choose ETFs if:

You are investing in a taxable brokerage account (tax efficiency matters)

You want maximum flexibility in buying and selling

You prefer to see real-time prices

You are making a lump-sum investment rather than recurring contributions

Choose index mutual funds if:

You are investing in a tax-advantaged account (401k, IRA) where tax efficiency is irrelevant

You want seamless automatic recurring investments

You prefer the simplicity of dollar-amount investing ($500/month) rather than share-based investing

Your employer's 401(k) only offers mutual funds (most do)

It honestly does not matter much if:

You are investing at Vanguard, Fidelity, or Schwab where both options have near-zero costs

You are a long-term buy-and-hold investor

Your time horizon is 10+ years

The Top Picks for 2025

S&P 500 trackers:

ETF: Vanguard S&P 500 ETF (VOO) — 0.03% expense ratio

Mutual Fund: Fidelity 500 Index Fund (FXAIX) — 0.015% expense ratio

Free option: Fidelity ZERO Large Cap Index Fund (FNILX) — 0.00%

Total US Market:

ETF: Vanguard Total Stock Market ETF (VTI) — 0.03%

Mutual Fund: Fidelity Total Market Index Fund (FSKAX) — 0.015%

Free option: Fidelity ZERO Total Market Index Fund (FZROX) — 0.00%

The performance difference between these options is negligible. Pick the one that fits your workflow best. What matters far more is that you invest consistently — see our guide on compound interest and use our Investment Calculator to model how your portfolio grows over time.

The Behavior Gap: The Real Enemy

According to Dalbar's 2024 Quantitative Analysis of Investor Behavior, the average equity fund investor earned just 16.54% in 2024 versus the S&P 500's 25.05% return — an 8.5 percentage point gap. This "behavior gap" comes from poorly timed buying and selling, not from choosing the wrong fund type.

The best investment vehicle is the one you will consistently invest in and leave alone. Whether that is an ETF, index mutual fund, or a mix of both, the discipline of regular investing matters far more than the wrapper. Check your salary percentile to ensure you are earning enough to invest, and start building wealth with our Investment Calculator.