If you've ever opened a Fidelity account and tried to pick a US stock index fund, you've probably stared at FSKAX and FXAIX wondering which one to buy. They look almost identical on the surface, with rock-bottom fees and broad US exposure, but they track different indexes and produce slightly different returns.
This guide compares FSKAX vs FXAIX side by side. By the end, you'll know which fund fits your portfolio, whether you should own both, and what each one is actually buying inside.
FSKAX vs FXAIX: The Quick Answer
FSKAX is the Fidelity Total Market Index Fund. It tracks the Dow Jones US Total Stock Market Index, which holds roughly 4,000 US companies of all sizes. FXAIX is the Fidelity 500 Index Fund, which tracks the S&P 500 and holds 500 of the largest US companies.
Both funds have an expense ratio of 0.015 percent, no minimum investment, and no transaction fee on Fidelity. The real difference is what they hold: FSKAX is the entire US market, while FXAIX is just the large-cap slice.
For most investors, the choice comes down to whether you want to own small and mid-cap companies in addition to the big names.
What FSKAX Actually Holds
FSKAX holds about 4,000 US stocks across large-cap, mid-cap, and small-cap segments. The largest holdings include the same mega-cap names you'd expect, like Apple, Microsoft, NVIDIA, Amazon, and Alphabet.
Because the fund is market-cap weighted, those mega-caps still drive most of the return. Roughly 85 percent of FSKAX's value sits in large-cap stocks, with about 10 percent in mid-caps and 5 percent in small-caps.
That small-cap and mid-cap exposure is the entire point of total market funds. Historically, smaller companies have outperformed large-caps over very long periods, though with higher volatility. FSKAX captures that potential without you having to pick the funds yourself.
What FXAIX Actually Holds
FXAIX tracks the S&P 500, so it holds exactly 500 of the largest US companies. These companies cover roughly 80 percent of the total US stock market by value.
The S&P 500 has stricter rules than a total market index. To be included, a company must be US-based, have a market cap above a set threshold, post positive recent earnings, and meet liquidity requirements. A committee at S&P Dow Jones Indices makes the final calls on inclusion.
This means FXAIX skips the smallest companies entirely. You're getting concentrated exposure to America's biggest and most profitable businesses, which has been a winning bet for the last 15 years.
Historical Performance Compared
Over long periods, FSKAX and FXAIX produce very similar returns. They share most of their holdings, and the large-caps that dominate both funds drive most of the performance.
The differences show up at the margins. In years when small-caps and mid-caps outperform large-caps, FSKAX edges ahead. In years when mega-caps run the show, FXAIX wins. Over the last 10 years, FXAIX has had a slight edge because mega-cap tech stocks have outperformed almost everything else.
Neither fund has a structural advantage that will persist forever. Historical returns are not a guarantee of future performance, and the gap between the two has rarely exceeded 1 to 2 percentage points in a single year.
Fees and Expense Ratios
Both funds charge an expense ratio of 0.015 percent per year. On a $10,000 investment, that's $1.50 per year. There is effectively no fee difference.
Fidelity does not charge transaction fees to buy or sell either fund on its own platform. Both funds also have a $0 minimum investment, which makes them accessible for new investors starting with small amounts.
Fidelity offers an even cheaper option, FZROX, which has a 0 percent expense ratio. The catch is that FZROX is not portable. If you ever leave Fidelity, you have to sell it, which can trigger a taxable event in a brokerage account. FSKAX and FXAIX both transfer to other brokerages cleanly.
Tax Efficiency Inside Each Fund
Both FSKAX and FXAIX are mutual funds, which means they sometimes distribute capital gains at year-end. Index mutual funds typically have low turnover, so distributions tend to be small.
For taxable brokerage accounts, ETF equivalents like ITOT (total market) or IVV (S&P 500) are slightly more tax-efficient because of how ETFs handle redemptions internally. In a Roth IRA or 401(k), tax efficiency doesn't matter, so FSKAX or FXAIX work just fine.
Consult a tax professional for personal advice, especially if you're moving large positions between accounts or fund families.
Should You Own Both FSKAX and FXAIX
Generally, no. Owning both creates massive overlap. The 500 stocks inside FXAIX are also inside FSKAX, so you'd be doubling up on the same large-caps without adding meaningful diversification.
Pick one based on your preference:
- Choose FSKAX if you want exposure to small and mid-cap US stocks in addition to the big names
- Choose FXAIX if you prefer a more concentrated bet on large-cap US companies and like benchmarking against the S&P 500
- Add a separate small-cap fund (like FSSNX) only if you want to overweight small-caps beyond what FSKAX gives you
Most long-term investors do fine with just one of these as their core US holding, paired with an international fund and bonds.
Where to Buy FSKAX or FXAIX
Both funds are Fidelity-branded mutual funds, but you don't have to use Fidelity to invest in similar exposure. If you want a commission-free brokerage with a clean app for buying ETFs and index exposure, Robinhood offers stocks, ETFs, options, and a Roth IRA with no minimums and no commissions. ETFs like ITOT (total market) or IVV (S&P 500) on Robinhood give you nearly identical exposure to FSKAX and FXAIX without leaving the app you already use.
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Where Each Fund Fits in a Portfolio
Both FSKAX and FXAIX work as the core US holding in a simple long-term portfolio. A common layout looks like:
- 60-70 percent in FSKAX or FXAIX (US stocks)
- 20-30 percent in an international fund like FTIHX or FZILX
- 10-20 percent in a bond index fund like FXNAX (more as you approach retirement)
This kind of three-fund portfolio is cheap, simple to manage, and tends to outperform most actively managed strategies over decades.
The choice between FSKAX and FXAIX is far less important than your savings rate, your time horizon, and whether you actually stay invested during downturns.
Frequently Asked Questions
Is FSKAX better than FXAIX?
Neither is objectively better. FSKAX offers broader diversification with about 4,000 holdings across all company sizes, while FXAIX concentrates on the 500 largest US companies. Returns over long periods are very similar, with FSKAX winning slightly when small-caps outperform and FXAIX winning slightly when mega-caps lead.
Can I hold both FSKAX and FXAIX in the same account?
You can, but it's usually redundant. The 500 stocks in FXAIX are already inside FSKAX. Holding both effectively overweights large-caps without adding new diversification. Pick one as your core US holding.
What's the difference between FSKAX and FZROX?
FZROX has a 0 percent expense ratio versus 0.015 percent for FSKAX, but FZROX is only available at Fidelity. If you ever transfer your account elsewhere, you'd have to sell FZROX first, which can trigger capital gains tax in a brokerage account. FSKAX is portable across brokerages.
Are FSKAX and FXAIX good for retirement accounts?
Yes. Both are widely considered solid core holdings for Roth IRAs, traditional IRAs, and 401(k)s. Their low expense ratios and broad diversification fit the long time horizons of retirement investing well. Past performance is not a guarantee of future results.
