Wealthy PotFSKAX vs FXAIX: Same Fee, Two Different Slices of the Market
Most comparisons of two index funds come down to cost. This one does not. FSKAX and FXAIX carry exactly the same expense ratio, 0.015%, both have no investment minimum, and both come from Fidelity. If you were hoping the cheaper one would settle it, nothing here will. The only real difference is what they own: FXAIX buys the S&P 500, and FSKAX buys the whole U.S. stock market.
The Short Answer
- Want the simplest one-fund way to own U.S. stocks? FSKAX. It holds the total market, so you never have to decide separately about mid-caps and small-caps. Nothing gets left out.
- Already hold small-cap or mid-cap funds, or your 401(k) only offers an S&P 500 option? FXAIX. The S&P 500 is the standard large-cap building block, and it is the more common choice inside workplace plans.
Because the fee is identical, there is no cost penalty for either answer. This is a question about how you want your portfolio put together, not about which fund is better run.
The Fee Is a Tie
Per Fidelity's own fund pages, both funds report a gross expense ratio of 0.015%. On a $100,000 balance that is $15 a year in each case. There is no fee argument to make, in either direction, and any page that tells you to pick one because it is cheaper is describing something that is not true.
Both also have a $0 minimum, so neither locks out a small starting balance. Fidelity charges no transaction fee to buy either one in a Fidelity account.
FSKAX vs FXAIX Side by Side
| Feature | FSKAX | FXAIX |
|---|---|---|
| Full name | Fidelity Total Market Index Fund | Fidelity 500 Index Fund |
| What it owns | The total U.S. stock market: large, mid and small caps | The S&P 500, large-cap U.S. companies |
| Index tracked | Dow Jones U.S. Total Stock Market Index | S&P 500 |
| Expense ratio | 0.015% | 0.015% |
| Minimum | $0 | $0 |
| Fund size | About $142 billion | About $859 billion |
| Wrapper | Mutual fund | Mutual fund |
What the Extra Stocks Actually Do
FSKAX owns everything FXAIX owns, plus several thousand mid-cap and small-cap companies that the S&P 500 leaves out. That sounds like a large difference and, measured by the number of holdings, it is.
Measured by money, it is not. Both funds weight their holdings by market capitalisation, which means the biggest companies take the biggest share. The S&P 500 already captures the large majority of total U.S. market value, so the thousands of additional smaller companies in FSKAX occupy a modest slice of the fund. The extra names change the fund's character at the edges; they do not change what dominates it.
That has a practical consequence worth being honest about: if you buy FSKAX expecting meaningful small-cap exposure, you will not get much. Investors who specifically want small-cap weight generally have to add a dedicated small-cap fund rather than rely on a total-market fund to deliver it.
Why Their Returns Look So Close
For the reason above, these two funds move together closely. Over any given stretch one will edge ahead of the other, depending on whether large caps or smaller companies are leading, and financial media will write that up as though a winner has emerged. Over a long horizon the gap between a total-market fund and an S&P 500 fund has historically been small relative to the gap between being invested and not being invested.
Be careful reading past performance here. A three- or five-year window that happens to cover a period of large-cap dominance will make FXAIX look better, and a different window will favour FSKAX. Neither tells you what comes next.
When Not to Switch
If you already hold one of these in a taxable brokerage account and it has gained value, selling to move into the other one realises a capital gain and generates a tax bill. You would be paying real money to swap between two funds with the same fee that behave almost identically. That is very rarely worth it.
Inside an IRA or 401(k) there is no tax consequence to switching, so if you prefer the other fund you can simply exchange. Even then, the reason to do it should be a considered decision about market coverage, not a recent performance chart.
One more case: if your workplace plan offers only FXAIX, that is a perfectly good core holding and you do not need to work around it. If you want broader coverage, the cleaner fix is usually adding an extended-market or small-cap fund alongside it, if your plan offers one.
Which One Fits You
Pick FSKAX if you want one fund to be your entire U.S. stock allocation and you would rather not think about whether mid-caps and small-caps are covered. It is the more complete single building block.
Pick FXAIX if the S&P 500 is what your plan offers, if you are deliberately building a portfolio out of separate large-, mid- and small-cap pieces, or if you simply prefer the most widely tracked U.S. benchmark.
Whichever you choose, the decision that matters far more than this one is how much you contribute and how long you leave it alone. Both of these funds are cheap, broad and well run. The difference between them is real but small.
FAQ
Is FSKAX or FXAIX cheaper?
Neither. Both report a 0.015% expense ratio, which works out to $15 a year on $100,000 in either fund.
Does FSKAX include the S&P 500?
Yes. FSKAX tracks the total U.S. stock market, which contains the S&P 500 companies plus thousands of smaller ones. Holding both funds together is largely duplicate exposure.
Should I own both FSKAX and FXAIX?
Generally no. They overlap heavily, so owning both mostly increases your large-cap weight rather than diversifying you. Pick one as your U.S. stock holding.
Which has performed better?
It depends entirely on the period you measure. Stretches when large companies lead favour FXAIX; stretches when smaller companies lead favour FSKAX. Past performance does not predict which will lead next.
Can I hold these outside Fidelity?
Fidelity mutual funds can sometimes be bought at other brokers, often with a transaction fee, and availability varies. If you want something you can move freely between brokers, an ETF is usually the simpler choice.
This article is for general information and is not investment advice. Fund figures were taken from Fidelity's published fund pages on 2026-09-21 and can change; confirm current figures before you invest.