For U.S. investors, choosing a low-cost index fund starts with choosing the right exposure and risk level—not finding the smallest expense ratio. Identify the market or asset class that fits your long-term plan, then compare funds tracking an appropriate index on diversification, total costs, tracking, and account fit. Review each fund’s current prospectus and shareholder report; low cost alone does not make a fund suitable.
Start with the investment you need, not the fund label
An index fund is a mutual fund or exchange-traded fund (ETF) that seeks to track a market index. You cannot buy the index itself: you buy shares in a fund that holds securities intended to follow it. The fund may own every security in its benchmark or a representative sample, and some funds use derivatives. The index’s rules—including which securities it includes and how it weights them—shape your actual exposure. The SEC explains these mechanics in its Investor Bulletin: Index Funds.
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Before comparing tickers, decide what role the investment should play: for example, exposure to a broad domestic stock market, a particular segment, or another asset class. Then consider how much investment risk fits your goals, timeframe, and ability to tolerate losses. Asset allocation is a separate decision from fund selection; a low-cost fund can still provide the wrong exposure for your plan. The SEC’s 2026 Investor.gov guidance notes that allocation depends on risk tolerance and timeframe.
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Index and exposure
Read the index name and its methodology, not just the fund name. Find out what market, geography, asset class, or segment it covers, how securities enter or leave the index, and how the index weights them. A broad index and a narrow or custom index can produce very different portfolios even when both funds call themselves index funds.
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Diversification and concentration
Check the fund’s actual holdings and compare them with other investments you already own. A fund can track an index and still be concentrated in a small number of securities or overlap substantially with another fund in your portfolio. The SEC cautions that custom-built “smart beta,” quantitative, and other non-traditional indexes can have unexpected overlaps or concentrations; its bulletin on non-traditional index funds advises understanding how an index is constructed.
Total costs, not just the expense ratio
Begin with the prospectus fee table and expense ratio. The expense ratio expresses annual fund operating expenses as a percentage of fund assets, but it is not necessarily the full cost of buying, owning, and selling the investment. Depending on the fund and account, also check for sales loads, transaction fees, brokerage commissions, account fees, bid-ask spreads for ETF trades, and portfolio trading costs. Some costs, including certain trading or securities-lending-related costs, may not appear in the expense ratio. The SEC’s fund and ETF fee bulletin warns that “zero expense” marketing does not mean an investment has no costs.
Fees matter because they leave less money invested to earn returns. In its 2025 fee bulletin, the SEC illustrates the effect with a hypothetical—not observed or forecast—scenario: $100,000 growing at 4% annually over 20 years under different annual fee assumptions. The SEC’s earlier index-fund bulletin makes the comparison conditional: “If the holdings of two funds have identical performance, the fund with the lower cost generally will generate higher returns for you.” That does not establish that two funds will have identical performance or that the cheapest candidate is the right one.
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Tracking and implementation
Find out whether the fund fully replicates its index or uses representative sampling, then review how closely its returns have tracked the benchmark after expenses. Differences can arise from fees, trading costs, sampling, and tracking error. Indexing does not guarantee exact benchmark performance, protect against market declines, or eliminate risk: a fund remains exposed to the securities in its index and may have less flexibility to respond when they fall. Past tracking is informative, not a promise of future results.
Compare mutual funds and ETFs in your account context
Mutual funds and ETFs can both offer diversified, passive exposure, and both have fees and expenses. Their trading mechanics and fee structures differ, so consider how you invest and what your brokerage account charges rather than assuming one type is always cheaper or better.
Taxes also depend on the account and circumstances. The SEC says ETF tax advantages have historically applied in some contexts, but there is no ETF-versus-mutual-fund tax difference when shares are held in tax-advantaged accounts such as a 401(k) or IRA. Check the SEC’s comparison of mutual funds and ETFs and consider your own tax situation; this is general education, not individualized tax advice.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Use current disclosures to make the final comparison
For each candidate, consult its current prospectus and shareholder report. Use the disclosures to verify the index, investment approach, fee table, risks, holdings information, and reported performance. FINRA’s Fund Analyzer can help compare fees and costs for certain mutual funds and ETFs, but it is a comparison tool rather than a personalized recommendation.
- What market exposure and risk level does this fund add to my plan?
- How is the index composed and weighted, and what do the fund’s holdings actually contain?
- How much do I expect to pay to buy, own, and sell the fund, including costs beyond its expense ratio?
- How does the fund implement its strategy, and how has it tracked its index after expenses?
- Does the fund’s structure fit my account’s trading and tax context?
The SEC’s Office of Investor Education and Advocacy suggests asking: “What fees and expenses can I expect to pay for buying, owning, and selling this fund?” It also recommends asking what specific risks the fund carries, how its index is determined, and how its strategy fits your investment goals.
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