Not all ETFs are index funds, and not all index funds are ETFs

An ETF (exchange-traded fund) is a container — a way to hold and trade investments. An index fund is a strategy — a fund that tracks a specific list of stocks or bonds. An ETF can hold an index strategy inside it, or it can hold something else entirely. A traditional mutual fund can also track an index. So the two terms describe different things, and they overlap but do not mean the same thing.

Think of it this way: all index-tracking ETFs are ETFs, but not all ETFs track an index. And all index-tracking ETFs are index funds, but not all index funds trade on an exchange like stocks do.

Key Takeaways

  • An ETF is a structure that trades on an exchange during market hours; an index fund is a strategy that mirrors a specific list of stocks or bonds.
  • An index-tracking ETF combines both: it trades like a stock but holds the same investments as an index fund would.
  • A traditional mutual fund can also track an index, but it trades once per day at the closing price, not throughout the day like an ETF.
  • Some ETFs do not track an index at all — they may hold actively managed strategies, bonds, commodities, or other assets.

How ETFs and index funds differ in structure

An ETF is a fund that trades on a stock exchange — you buy and sell shares of it during market hours at whatever price the market sets at that moment, just like you would with a stock. A traditional index fund is usually a mutual fund, which means you buy it directly from the fund company, and the price is set once per day after the market closes.

An index fund — whether it is an ETF or a mutual fund — holds a basket of investments designed to match the performance of a specific index. An index is a published list: the S&P 500 is 500 large U.S. companies, the Nasdaq-100 is 100 large technology and growth companies, the Bloomberg U.S. Aggregate Bond Index tracks thousands of bonds. The fund buys those same investments in the same proportions, so when the index goes up or down, the fund does too.

Because an index fund straightforward copies a list rather than paying a manager to pick stocks, index funds typically charge lower fees than actively managed funds, where a manager decides what to buy and sell.

When an ETF tracks an index

Many ETFs are index funds. They track a published index and trade on an exchange. Examples include the Vanguard S&P 500 ETF (ticker: VOO), which tracks the S&P 500, and the iShares Core U.S. Aggregate Bond ETF (ticker: AGG), which tracks the Bloomberg U.S. Aggregate Bond Index. When you buy these, you own a piece of a fund that holds all 500 stocks (or thousands of bonds) in the same weights as the index.

The advantage of an index-tracking ETF over a traditional index mutual fund is that you can trade it during the day at any price, and you may pay a smaller bid-ask spread (the difference between the buy and sell price) because ETFs are often more liquid. The disadvantage is that you may pay a commission to buy or sell it, depending on your brokerage, whereas many brokerages let you buy mutual funds commission-free.

When an ETF does not track an index

Some ETFs do not track an index at all. An actively managed ETF holds investments that a manager picks and changes over time, trying to beat the market or follow a specific strategy. A bond ETF might hold individual bonds that the manager selects rather than tracking a bond index. A commodity ETF might hold gold, oil, or other raw materials. A leveraged ETF uses borrowed money to amplify gains (and losses). None of these are index funds, even though they are ETFs.

The key difference is that an index fund, by definition, follows a published index. If a fund manager is making decisions about what to buy and sell, it is not an index fund, regardless of whether it trades on an exchange.

When an index fund is not an ETF

Many index funds are traditional mutual funds that do not trade on an exchange. You might own the Vanguard 500 Index Fund (ticker: VFIAX), which tracks the S&P 500 just like the Vanguard S&P 500 ETF does, but you buy it directly from Vanguard at the closing price each day. Fidelity, Schwab, and other fund companies offer similar index mutual funds.

The main practical difference is when and how you can trade. With a mutual fund, you place an order during the day, but the transaction happens at the closing price. With an ETF, you see the price in real time and can trade it like a stock. For long-term investors who buy and hold, this difference matters less. For people who trade frequently, the ability to trade during the day may matter more.

Fees and tax treatment

Index-tracking ETFs and index mutual funds usually charge similar annual fees because they are both following a published list rather than paying a manager to pick investments. You might see expense ratios of 0.03% to 0.20% per year for either type. Actively managed ETFs typically charge higher fees than index-tracking ones.

In a taxable account, ETFs often have a tax advantage over mutual funds because of how they are structured. When investors sell shares of a mutual fund, the fund may have to sell investments to raise cash, which can trigger capital gains taxes for all remaining shareholders. ETFs use a different mechanism (called in-kind redemption) that usually avoids this. In a retirement account like a 401(k) or IRA, this tax difference does not matter because the account itself is tax-deferred.

How to tell what you are looking at

Check the fund's prospectus or fact sheet. It will state whether the fund tracks an index or is actively managed. If it says "tracks the S&P 500" or "seeks to replicate the performance of the [index name]," it is an index fund. If it says "seeks to outperform" or "actively managed," it is not.

You can also look at the fund's ticker and name. Index funds often say "index" in the name (like "Vanguard Total Stock Market Index Fund") or use abbreviations like "VOO" or "VTI." Actively managed funds may not signal this in the name, though it is not a reliable rule.

Frequently Asked Questions

Can I buy an index fund that is not an ETF?

Yes. Most fund companies offer index mutual funds that track major indexes like the S&P 500 or total bond market. You buy them directly from the fund company or through a brokerage, and they trade once per day at the closing price. Vanguard, Fidelity, and Schwab all offer index mutual funds alongside their index ETFs.

Do index ETFs and index mutual funds perform differently?

Not meaningfully. Both track the same index, so they should move in lockstep. The difference is in how you trade them (during the day for ETFs, once per day for mutual funds) and, in taxable accounts, how they handle capital gains. Over time, a low-cost index ETF and a low-cost index mutual fund tracking the same index should deliver nearly identical returns.

Why would I buy an actively managed ETF instead of an index ETF?

An actively managed ETF may appeal to you if you believe a particular manager or strategy can outperform the market, or if you want exposure to something that is not covered by a major index (like a specific sector or international region). The trade-off is higher fees and no may provide of better returns.

Are all low-cost ETFs index funds?

No. Some actively managed ETFs charge low fees too. Check the prospectus to see whether the fund tracks an index or is actively managed. The expense ratio alone does not tell you which it is.