Index Funds vs ETFs: What’s the Difference?

Index funds and ETFs let you buy a slice of the market without picking individual stocks.

Both can offer diversification, low fees, and a simple way to invest for the long term.

But here is a common point of confusion: “index fund” and “ETF” are not competing categories. One describes what a fund invests in; the other describes how its shares are traded. A single fund can be both.

Understanding that difference makes funds much easier to compare, and helps you choose investments that fit your brokerage account, IRA, or 401(k). For the full primer on the strategy itself, see our guide to what an index fund is and how it works.

What Is an Index Fund?

An index fund is an investment fund that seeks to replicate, or track, the performance of a market index.

An index is a basket of securities that meet specific criteria.

The S&P 500 Index, for instance, tracks roughly 500 of the largest U.S. companies by market value.

Other indexes track small-company stocks, foreign stocks, bonds, or even a single sector.

An index fund usually holds the securities in an index, or a representative sample of them, and attempts to track that index before fees and other costs.

Because the securities are largely selected mechanically, these funds are considered passively managed.

As the SEC notes, index funds can take the form of either mutual funds or exchange-traded funds.

What Is an ETF?

ETF stands for exchange-traded fund.

An ETF is a pooled investment vehicle that owns a portfolio of securities, such as stocks and bonds.

Its defining feature is that it trades on a stock exchange.

Shares of an ETF can be bought and sold throughout the trading day, just like individual stocks, and the price moves constantly in response to supply and demand.

Some ETFs track market indexes. Others are actively managed, like many mutual funds, meaning the fund managers choose the holdings based on their own outlook.

An ETF can be:

• a U.S. stock market index fund
• an international stock index fund
• a bond index fund
• an actively managed fund
• a specialized or concentrated fund

As the SEC points out, ETFs can vary greatly in diversification, strategy, expense ratios, and risk.

Index Funds vs ETFs: Two Very Different Questions

Think of two separate questions: “What am I investing in, and how is it chosen?” versus “How do I buy and sell shares of this investment?”

An index fund answers the first question. An ETF answers the second.

Index fund: an investment strategy that tracks a selected index.

ETF: a fund structure whose shares trade on an exchange.

A single fund can be both an index fund and an ETF. An S&P 500 index ETF, for example, tracks the Standard & Poor’s 500 Index and trades on an exchange.

For most investors, the more useful comparison is between an index mutual fund and an index ETF. For the wider picture of how these pieces fit together, see our guide to stocks vs ETFs vs mutual funds.

Index Mutual Funds vs Index ETFs: Quick Comparison

In general, either option can be a good way to build long-term wealth through diversified investing.

These differences become more meaningful when you consider how transactions actually work.

How You Buy and Sell

Index Mutual Funds

Traditional index mutual funds are generally priced once per business day.

The price is based on the fund’s net asset value, or NAV.

NAV is the value of the fund’s assets minus its liabilities, divided by the number of shares outstanding.

If you place a purchase or redemption at 11 a.m., your transaction takes place at the next calculated NAV, usually after the market closes.

You cannot know the exact price in advance.

For someone making monthly investments over 20 or 30 years, this usually does not matter.

Index ETFs

Index ETFs trade throughout the day, like individual stocks.

Prices change constantly, based on supply and demand.

Index ETFs also allow different order types, depending on the brokerage.

Suppose an ETF is trading at $105. You could place a market order (buying at the next available price) or a limit order (setting the maximum price you are willing to pay).

That flexibility is a plus, but it can also lead to overthinking.

Investors drawn to the idea of a simple, low-cost fund sometimes fall into the trap of trading too much.

The SEC notes that an ETF trades at a market price, which can differ from the fund’s NAV.

Investment Costs

Index ETFs and index mutual funds both have operating expenses, usually expressed as an expense ratio.

If a fund charges 0.10 percent, that works out to roughly $10 per year for every $10,000 invested, assuming a stable balance.

If the fund charges 0.50 percent, that is roughly $50 on the same $10,000.

Over decades, the difference in expenses can be significant.

A Simple Fee Comparison

Suppose two hypothetical funds hold similar investments and earn a 7 percent annual return before expenses.

Fund A has an expense ratio of 0.05 percent. Fund B has an expense ratio of 0.50 percent.

Both start with a $10,000 investment and make no additional contributions.

If fees come straight out of the return, Fund A keeps a 6.95 percent net return and Fund B keeps 6.50 percent. Over 30 years, that small gap compounds into a meaningful difference:

In this simplified, hypothetical example, the higher expense ratio costs roughly $9,000 over 30 years. These are rounded estimates, not forecasts.

Neither fund is inherently better; it depends on the specific offerings. Compare expense ratios and other costs for the funds you are considering.

The SEC advises investors to weigh a fund’s expenses and transaction costs. Small differences in fees compound over time.

ETFs Have Bid-Ask Spreads

When you buy or sell an ETF, you are trading shares with another investor on the exchange.

The ETF’s price has two sides: the bid and the ask.

The bid is the highest price a buyer is currently willing to pay. The ask is the lowest price a seller is willing to accept.

The difference between the bid and the ask is the bid-ask spread, and crossing it is effectively a cost of trading.

Suppose you see the following on a stock screen:

Bid: $49.98
Ask: $50.02

The spread is $0.04 per share.

If you bought a share at the ask price and immediately sold it back at the bid price, you would lose about 4 cents per share before any other commissions or fees, assuming those prices held and your orders filled there.

ETFs with a lot of liquidity usually have small spreads, while thinly traded ETFs tend to have wider spreads.

The SEC describes the bid-ask spread as an important cost associated with ETFs.

For a long-term investor, this is usually a minor consideration, especially with a highly liquid ETF. For an active trader, spreads can add up.

Tax Efficiency

Taxes are another consideration, particularly if you hold funds in a taxable brokerage account.

Mutual funds can realize capital gains when they sell securities inside the fund.

Those gains are then distributed to shareholders, which can create a tax bill even if you did not sell any of your own shares.

ETFs generally have a tax advantage over mutual funds.

Because of the way ETFs are created and redeemed, it is generally easier to defer capital gains, so ETFs have tended to be more tax-efficient than mutual funds in taxable accounts.

There are several caveats.

Index mutual funds tend to be tax-efficient too, because they generally trade less than actively managed funds.

ETFs can still realize capital gains.

And selling either type of investment at a profit in a taxable account creates a taxable capital gain.

What About IRAs and 401(k)s?

When you hold investments inside a retirement account, many of the tax issues that apply in a taxable account no longer apply.

Transactions inside a retirement account are governed by the tax rules for that account.

Selling a fund inside an IRA, for instance, does not generally create a capital gain or loss.

The tax treatment depends on whether you have a traditional IRA or a Roth IRA, among other factors.

The SEC notes that the usual ETF tax advantage over mutual funds does not apply inside tax-deferred or tax-exempt accounts such as IRAs and 401(k)s.

Minimum Investment Requirements

An index mutual fund might have a minimum investment.

Some funds have no minimum. Others might require $1,000, $3,000, or another amount.

Minimums vary from fund to fund.

ETFs traditionally had an effective minimum too, because you needed enough money to buy at least one share.

If an ETF share cost $300 and you wanted to invest only $100, you could not buy in. You would have needed at least $300 for one full share.

Fractional share purchases changed that.

Many brokerages now let eligible investors buy fractional ETF shares, so it can be possible to invest $50 in an ETF even when a full share costs several times that.

Check your brokerage’s rules before making a decision based on minimums.

Automatic Investing

Index mutual funds have traditionally made automatic monthly investing easy.

Suppose you want to invest $400 every month.

You could set up a recurring $400 contribution, and the fund would buy fractional shares so the full $400 is invested.

ETFs can also be bought on an automatic, recurring basis through brokers that offer the feature, though availability varies.

If you want to make monthly contributions, check whether the fund and your brokerage support automatic purchases.

Investing a fixed amount on a regular schedule is known as dollar-cost averaging, and that kind of convenience is a key factor in long-term investing.

Diversification Depends on the Index

If you are comparing an index ETF and an index mutual fund that track the same index, diversification is generally not the issue. The two funds would have a very similar risk profile.

If one fund tracks the S&P 500 and the other tracks a technology-sector ETF, diversification matters much more.

The technology ETF could be a highly concentrated position compared with the more broadly diversified index fund.

Even within the S&P 500, diversification across sectors is limited, because the index is weighted toward its largest companies.

Investor.gov warns that some index funds have narrow or unusual benchmarks, which can increase their risk.

Look at the fund’s holdings and the index it tracks before assuming that any index fund is well diversified.

Performance Can Be Nearly Identical

Assume two funds track the same index, one an index mutual fund and the other an index ETF.

If both track the index closely, their performance should be nearly identical, apart from expenses and other small factors.

The structure of the fund does not itself produce outperformance. Expenses, tracking error, and other factors do.

When comparing funds, look at the expense ratios, the tracking difference, and the other features of each fund.

The underlying performance of the two should be nearly identical, based on the index they track.

Which Is Better for Beginners?

An index fund or an ETF can be a good choice for a beginning investor.

Which one is better often depends on the type of account and how comfortable you are with the buying process. It is one of the first decisions many people face when they start investing.

When an Index Mutual Fund Might Be Helpful

An index mutual fund can be a good option when automatic monthly contributions are available, round-dollar purchases are convenient, and you do not want to think about the price of an ETF.

It might also be the default if your 401(k) plan only offers index mutual funds.

When an Index ETF Might Be a Good Option

An index ETF might be preferable if you want the flexibility of trading like an individual stock, you want access to a particular low-cost fund, or you want the potential tax advantages of an ETF.

It can also make sense if your brokerage has eliminated purchase commissions and allows automatic, recurring purchases of fractional shares.

A Practical Example: Investing $500 Every Month

Suppose you want to invest $500 a month in a diversified U.S. stock index.

You have two options:

Option A: an index mutual fund that tracks the index, with a 0.05 percent expense ratio.
Option B: an index ETF that tracks the same index, with a 0.05 percent expense ratio.

Assume both track the index closely and your brokerage charges no commission.

In that case, the long-term prospects of the two funds would be nearly identical.

With Option A, you could set up a $500 monthly purchase through the mutual fund.

With Option B, your brokerage could buy $500 worth of shares each month, if fractional ETF purchases are available.

The ETF would add the small risk and cost of the bid-ask spread, while the mutual fund would use the next NAV for purchases.

The larger influences on long-term results would be the behavior of the market, your ability to keep investing, your expenses, and any tax implications of the investment.

Common Misunderstandings

“All ETFs Are Index Funds”

False. Many ETFs are actively managed rather than passively tracking an index. The manager of an actively managed ETF chooses the fund’s investments.

“All Index Funds Are Mutual Funds”

False. An index fund can also take the form of an ETF.

“ETFs Always Have Lower Fees”

False. Expenses vary by fund, and some index mutual funds have extremely low expense ratios.

“ETFs Are Always More Diversified”

False. An ETF can be highly concentrated, depending on what it holds.

“Index Funds Cannot Lose Money”

False. An index fund tracks an index, which can decline in value. A broad stock index still carries market risk, and it can fall sharply during a bear market.

How to Choose an Index Mutual Fund or Index ETF

Before buying, it helps to answer a few questions:

1. Which market index do I want to invest in?
2. Does the fund track that index closely?
3. What does the fund hold, and how concentrated are its largest positions?
4. What is the expense ratio?
5. Are there commissions, bid-ask spreads, or other costs?
6. Can I set up automatic purchases?
7. Will I hold the investment in a taxable account, an IRA, or a 401(k)?
8. Does the investment fit my long-term allocation plan?

These questions put the funds in perspective.

Rather than comparing one label with another, compare what is actually inside each fund.

The Bottom Line

Index funds and ETFs overlap.

An index fund tracks an index; an ETF is a fund whose shares trade on an exchange like a stock.

An ETF can also be an index fund.

For most investors, the more relevant comparison is between an index mutual fund and an index ETF.

Both can provide broad diversification and a simple way to invest in stocks or bonds.

The differences mainly come down to trading, transaction costs, automatic purchases, minimums, and taxes.

The considerations that matter most for any investment are what the fund holds, what it costs, and how it fits into your long-term plan.

Related Guides

• What Is an Index Fund and How Does It Work?
• Stocks vs ETFs vs Mutual Funds: Understanding the Differences
• Asset Allocation Explained: How Stocks, Bonds and Cash Fit Together
• Dollar-Cost Averaging Explained: How Regular Investing Works
• How Compound Interest Works: Why Time Matters More Than You Think
• Investing for Beginners: A Practical Introduction

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