There is no single "best" index fund — the right choice depends on what you are investing in, how much you pay in fees, and what your goals are

An index fund tracks a specific group of stocks or bonds — the S&P 500, the total U.S. stock market, international stocks, or bond indexes. Different funds track different indexes, and the same index can be tracked by multiple funds from different companies. The "best" fund for you is the one that tracks an index matching your investment plan, charges the lowest fees, and is offered through an account you already have or can open.

This matters because two funds tracking the same index can have different costs. A fund charging 0.03% per year costs far less over time than one charging 0.50% per year, even though both hold the same stocks. Over 30 years, that difference compounds into thousands of dollars in your pocket instead of the fund company's.

Key Takeaways

  • The index a fund tracks matters more than the fund company — decide first whether you want U.S. stocks, international stocks, bonds, or a mix, then compare funds that track that same index.
  • Expense ratios (the annual fee as a percentage of your investment) vary widely for the same index, so compare the actual percentage before opening an account.
  • The lowest-cost funds tracking major indexes are typically offered by Vanguard, Fidelity, and Schwab, though your employer retirement plan or existing brokerage may offer competitive options.
  • A fund's size matters — very small funds sometimes close and force you to move your money, while extremely large funds are stable but may be harder to buy or sell quickly.
  • Tax efficiency (how much of your gains go to taxes versus staying invested) varies by fund type and account, so holding index funds in a traditional IRA or 401(k) often saves more than holding them in a regular brokerage account.

Comparing funds that track the same index

Start by deciding what index you want to track. The S&P 500 (500 large U.S. companies) is the most common choice for a core holding. Other common options are the total U.S. stock market (which includes mid-size and small companies), international developed markets, emerging markets, and bond indexes like the Bloomberg U.S. Aggregate Bond Index.

Once you know the index, search for funds tracking it. You will find multiple options. Compare the expense ratio — the percentage you pay annually. For a U.S. stock index fund, expense ratios typically range from 0.03% to 0.20%. For bond funds, they range from 0.03% to 0.15%. For international or emerging market funds, they may be slightly higher, from 0.08% to 0.30%.

Calculate the actual dollar cost. If you invest $10,000 in a fund with a 0.03% expense ratio, you pay $3 per year. The same $10,000 in a 0.50% fund costs $50 per year. Over 20 years at 7% annual returns, that difference grows to roughly $2,000 in lost gains. The lower-cost fund is almost always the better choice when both track the same index.

Where to find low-cost index funds

Vanguard, Fidelity, and Schwab offer some of the lowest expense ratios on index funds. Vanguard's S&P 500 index fund (ticker VFIAX for brokerage accounts, VTSAX for total market) charges 0.03%. Fidelity's FSKAX (total U.S. market) and FXAIX (S&P 500) also charge 0.03%. Schwab's SWTSX (total U.S. market) charges 0.03%.

However, you may not have access to these directly. If you have a 401(k) through your employer, check what index funds your plan offers. Many employer plans offer Vanguard, Fidelity, or T. Rowe Price index funds at competitive rates. If your employer plan's lowest-cost S&P 500 fund charges 0.05% or less, it is likely a good choice for that account.

If you have an IRA or brokerage account you control, you can open an account at Vanguard, Fidelity, or Schwab directly and buy their lowest-cost funds. You do not need to use the same company for every account — many people hold a 401(k) through their employer and an IRA at a different company.

Fund size and stability

A fund's total assets under management matter for stability. A fund with $1 billion or more in assets is unlikely to close. A fund with $50 million or less is at higher risk of being shut down by the fund company, which forces you to move your money to another fund (usually without tax consequences, but it is an inconvenience).

Very large funds — those with $50 billion or more — are extremely stable but may have slightly wider bid-ask spreads if you are buying or selling small amounts. For most investors, this is not a practical concern. The trade-off between a fund with $2 billion in assets and one with $20 billion is negligible.

Check a fund's assets under management on the fund company's website or on financial data sites like Morningstar. If a fund you are considering has less than $100 million, look for an alternative tracking the same index.

Tax efficiency in different account types

Index funds are already tax-efficient compared to actively managed funds because they trade less frequently. However, the account type matters more than the fund choice.

In a traditional 401(k) or traditional IRA, you do not pay taxes on gains until you withdraw money in retirement. In a Roth IRA, you do not pay taxes on gains ever. In both cases, the fund's internal tax efficiency is less important because the account itself shields you from taxes.

In a regular brokerage account, you pay taxes on dividends and capital gains every year. Index funds produce fewer taxable events than actively managed funds, but some index funds are structured to minimize taxes further. ETF versions of index funds (which trade like stocks) are often more tax-efficient than mutual fund versions in taxable accounts. If you are investing outside a retirement account, ask whether the fund company offers an ETF version of the same index.

Index funds versus ETFs tracking the same index

Many index funds come in two versions: a mutual fund and an ETF (exchange-traded fund). Both track the same index and have the same expense ratio. The differences are small for most investors.

Mutual funds settle the day after you buy or sell. ETFs trade throughout the day like stocks, so you see the price change in real time. ETFs may have a small bid-ask spread (the difference between the buy and sell price), while mutual funds do not. For large investments, this spread is negligible. For very small investments, it may cost a few dollars.

Mutual funds require a minimum investment (often $1,000 to $3,000), while many ETFs can be bought one share at a time. If you are starting with a small amount, an ETF may be easier. If you are investing a large lump sum or setting up automatic monthly contributions, the difference does not matter.

Common mistakes when choosing an index fund

The biggest mistake is choosing based on past performance. An index fund that outperformed last year is not likely to outperform next year — it is tracking the same index as its competitors. Past performance tells you nothing about which fund to pick.

Another mistake is choosing a fund based on brand recognition. A well-known fund company is not necessarily cheaper. Always compare expense ratios directly, even between companies you have heard of and ones you have not.

A third mistake is holding too many index funds tracking overlapping indexes. If you own both a total U.S. stock market fund and an S&P 500 fund, you are holding the same 500 large companies twice. One fund is enough for your U.S. stock exposure. Simplicity reduces confusion and keeps your costs down.

Frequently Asked Questions

Should I choose an index fund based on which one performed best last year?

No. Index funds tracking the same index will have nearly identical returns over time. Last year's winner may lag this year. The only meaningful comparison is the expense ratio — the annual fee. A cheaper fund tracking the same index will outperform a more expensive one over the long term, all else equal.

What is the difference between an index fund and an ETF?

An ETF is a type of index fund that trades like a stock throughout the day, while a mutual fund index settles the next day. Both can track the same index and charge the same fee. ETFs may be easier to buy in small amounts, while mutual funds may be easier for automatic monthly contributions. The choice between them is minor compared to choosing the right index.

Can I hold index funds from different companies in the same account?

Yes. You can hold a Vanguard fund, a Fidelity fund, and a Schwab fund in the same brokerage account if that account allows it. However, most people do not need to. Picking one low-cost provider and staying there simplifies record-keeping and may reduce trading costs.

Is a smaller index fund riskier than a larger one?

The risk of the investments inside the fund (stocks or bonds) is the same regardless of fund size. However, a very small fund (under $50 million) may be closed by the fund company, forcing you to move your money. A fund with $500 million or more is stable. Size affects convenience, not investment risk.

Should I choose an index fund based on its dividend yield?

No. Two index funds tracking the same index will have nearly identical dividend yields because they hold the same stocks. A higher yield does not mean better performance — it may just mean the fund reinvests dividends differently or has a different share price. Focus on the index and the expense ratio, not the yield.