Same Fee, Same Firm, One Real Difference

FXAIX and FSKAX are Fidelity’s two conventional core index funds, and they share almost everything. Both charge 0.015% a year, both have no minimum investment and both can generally be moved to another broker. The one real difference is what they hold: FXAIX tracks the S&P 500, about 500 of the largest US companies, while FSKAX holds the entire US stock market, including thousands of mid-sized and smaller companies.

Because the fees are identical, this is the cleanest test there is of a simple question: is it better to own the largest US companies or the whole market? FRC’s calculations show that from 2012 to 2025, FXAIX finished ahead in 10 of 14 years, and $10,000 in it grew to about $4,070 more than the same amount in FSKAX.

That does not make FXAIX the better fund for everyone. This guide shows every year side by side, explains why the results differed, and helps you decide which approach suits your plans.

Start Here: Learn to Choose Funds With Confidence

Financial Regulation Courses (FRC) has no relationship with Fidelity and no stake in what you buy. FRC sells no investments, manages no money and earns no commission, so this comparison is explained impartially.

ETF Investing for Beginners explains how index funds and ETFs work, what they cost and how to compare them. ETF Investing Strategies teaches you to build a complete portfolio around funds like these, and Advanced ETF Investing covers rebalancing, tax placement and risk in depth.

FRC’s programs and memberships are open to everyone. Every chapter ends with an assessment you must pass before moving on, and FRC’s AI teachers present video lessons in the FRC dictionary, each reviewed by qualified finance and compliance professionals before it is published.

FXAIX vs FSKAX at a Glance

The two funds follow independent, widely used indexes licensed from outside providers. That is one reason both can generally be transferred to another broker without being sold, unlike Fidelity’s ZERO funds.

FRC’s guide to Fidelity index funds compares these two with FNILX and FZROX, Fidelity’s two zero-fee alternatives.

What FXAIX Holds

FXAIX tracks the S&P 500, an index of around 500 of the largest US companies chosen by an independent committee. These are the household names across technology, healthcare, finance and consumer goods.

The index is weighted by company size, so the biggest firms make up a large share of the fund. The S&P 500 covers roughly 80% of the value of the US stock market, which means FXAIX already gives you most of the market’s weight in one holding.

FXAIX leaves out the mid-sized and smaller companies that make up the remaining 20% or so. For investors who want a simple, focused core holding, that is exactly the point.

What FSKAX Holds

FSKAX holds everything FXAIX holds, plus thousands of mid-sized and smaller US companies, giving it around seven times as many holdings in total. It is designed to capture almost the entire US stock market in a single fund.

Because the market is weighted by size, the S&P 500 companies still make up around four-fifths of FSKAX’s value. The smaller companies add breadth, but they only move the fund’s results at the margins.

Supporters of FSKAX value that breadth. It means owning every part of the US market, including the smaller companies that may one day become tomorrow’s giants.

Year-by-Year Returns

FRC compared the calendar-year total returns of both funds, with dividends reinvested, for every full year from 2012 to 2025.

FXAIX finished ahead in 10 of the 14 full years. FSKAX led in 2012, 2013, 2016 and 2020, all years when smaller US companies rallied strongly.

The largest single gap came in 2021, when FXAIX led by just over 3 percentage points as the biggest technology companies surged. The next largest came in 2020, when FSKAX led by 2.4 points as smaller companies rebounded from the COVID crash.

What $10,000 Became

Over 14 years, the S&P 500 fund finished about $4,070 ahead on a $10,000 investment. With identical fees, that gap came entirely from the difference in what each fund held.

FSKAX was also slightly more volatile. It had both the higher best year and the deeper worst year, because smaller companies tend to swing further in both directions.

Returns Over 3, 5 and 10 Years

Many investors look at returns over standard periods rather than single years. FRC calculated what $10,000 became over each period to the end of 2025.

FXAIX led over every one of these periods. The widest yearly gap was over five years, a stretch dominated by the largest technology companies, while over 10 years the gap narrowed to about half a percentage point a year.

These figures are history, not a forecast. Double-digit yearly returns over a decade are well above the long-run average for US stocks, so it would be unwise to plan on them continuing.

Investing Every Year Instead

Most people invest regularly rather than all at once. FRC also modeled $6,000 invested at the start of each year from 2012 to 2025, $84,000 in total.

The regular investor in FXAIX ended about $13,800 ahead. The larger gap reflects how strongly large companies led in the later years, when the balance was biggest.

The Two Halves Tell Different Stories

Splitting the 14 years into two halves shows how much the result depends on the period you choose.

From 2012 to 2018, the two funds finished almost level, separated by less than $400 on $10,000. Most of FXAIX’s lead built up after 2019, when a small group of very large technology companies drove much of the market’s gains.

This is worth knowing before you decide. The recent lead of the S&P 500 reflects a particular kind of market, and there have been long periods in the past, such as the early 2000s, when smaller US companies did better than the largest ones.

How Closely Do They Move Together?

FRC measured the correlation of the two funds’ yearly returns from 2012 to 2025 at about 0.996, where 1.0 would mean they moved in perfect lockstep. The average gap between them was about 1.1 percentage points a year.

In practice, the two funds rise and fall together. In every losing year in this period, both lost money, and in every strong year, both gained.

That is why owning both adds very little. Holding FXAIX and FSKAX together mostly means owning the same large companies twice, which does not add real diversification.

The Fees Are Identical

Both funds charge 0.015% a year, which works out to about $1.50 for every $10,000 invested. On $100,000, that is about $15 a year for either fund.

Because the cost is the same, fees play no part in this decision. That makes FXAIX vs FSKAX different from comparisons involving Fidelity’s ZERO funds, where a zero fee is traded for being tied to Fidelity.

FRC’s comparison of FXAIX vs FZROX looks at that trade-off, including what it could cost in tax to leave Fidelity with a ZERO fund in a taxable account.

Both Funds Are Portable

FXAIX and FSKAX can both generally be transferred to another brokerage in kind, without being sold. That flexibility is one of their main advantages over the ZERO funds.

If you ever move your account, an in-kind transfer avoids selling and therefore avoids triggering capital gains tax in a taxable account. At some other brokers, holding or buying Fidelity mutual funds may involve a transaction fee, so check before you move.

Your brokerage account also carries its own layers of protection. FRC’s guide to whether Fidelity is safe explains how a Fidelity account is protected, and which risks no protection covers.

Which Account Are You Using?

Both funds are mutual funds, which can occasionally distribute capital gains that are taxed in a taxable account even if you did not sell anything. Index mutual funds have low turnover, so these distributions tend to be small, but they can happen.

Inside a retirement account, such as an IRA or a workplace plan, that does not matter, because nothing is taxed each year. In a taxable account, an S&P 500 or total market ETF may be slightly more tax-efficient, and FRC’s guide to where to hold ETFs explains which funds suit which accounts.

Many workplace plans offer an S&P 500 index option, sometimes alongside a total market option. FRC’s guide to how a 401(k) works explains how to read your plan’s fund menu.

Which One Fits Your Situation?

These are starting points, not recommendations. Over long periods, the two approaches have produced similar results, and the difference in any decade has depended on whether large or small companies led.

S&P 500 or Total Market: What Each Side Argues

Supporters of the total market approach argue that owning everything is the purest form of diversification. You never have to guess which companies will grow, because you own them all from the start.

Supporters of the S&P 500 argue that the largest companies already make up most of the market, so adding thousands of small ones changes results only slightly while adding a little volatility. They also point to the S&P 500’s long track record and its familiarity.

Both arguments are reasonable, and the data supports neither side permanently. FRC’s comparison of VOO vs VTI looks at the same choice through the two most popular ETFs.

What Warren Buffett Recommended

Warren Buffett is often quoted in this debate. In his 2013 letter to Berkshire Hathaway shareholders, he wrote that his instructions for the money left to his wife were to put 10% in short-term government bonds and 90% in a very low-cost S&P 500 index fund.

That makes the S&P 500 the index most closely associated with his advice, and FXAIX is one low-cost way to own it. Buffett’s broader point, though, was about keeping costs low and staying invested, which applies equally to a total market fund like FSKAX.

What If You Don’t Invest at Fidelity?

The same choice exists everywhere. S&P 500 ETFs and total market ETFs are available at almost every US broker, often at similarly low costs.

FRC’s VOO performance history and VTI performance history show how an S&P 500 fund and a total market fund have performed in every year since they launched, including the 2008 financial crisis for VTI.

Whichever approach you choose, international stocks and bonds add something a US fund does not. A simple mix of US stocks, international stocks and bonds is the idea behind the three-fund portfolio.

Questions People Ask

Is FXAIX or FSKAX better? Neither is better in general. FXAIX holds the largest US companies, while FSKAX holds the whole US market, and both charge the same 0.015% fee.

Why did FXAIX beat FSKAX? Because the largest US companies outperformed smaller ones in most years since 2012, especially after 2019. FSKAX led in years when smaller companies rallied, such as 2013 and 2020.

Do FXAIX and FSKAX have the same fees? Yes. Both have an expense ratio of 0.015%, which is about $1.50 a year for every $10,000 invested.

Does FSKAX include the S&P 500? Yes. FSKAX holds all the companies in the S&P 500 plus thousands of smaller ones, and the S&P 500 companies make up most of its value.

Should I own both FXAIX and FSKAX? Usually there is little benefit, because they overlap heavily and their returns are closely correlated. International stocks or bonds add more variety than a second US fund.

Can I transfer FXAIX or FSKAX to another broker? Generally yes. Both can usually be moved in kind, without selling, though other brokers may charge a fee to hold or buy them.

Is FSKAX more volatile than FXAIX? Slightly. From 2012 to 2025, FSKAX had a deeper worst year and a larger fall in early 2020, because smaller companies tend to swing further.

Is FSKAX the same as FZROX? Both hold the whole US market, but FSKAX follows an outside index, charges 0.015% and is portable, while FZROX follows Fidelity’s own index, charges nothing and can only be held at Fidelity.

What is the average 10-year return of FXAIX? From 2016 to 2025, FXAIX returned about 14.8% a year with dividends reinvested, turning $10,000 into about $39,780. FSKAX returned about 14.2% a year over the same period.

What is Warren Buffett’s favorite index fund? Buffett has recommended a very low-cost S&P 500 index fund, most famously in his 2013 shareholder letter. FXAIX is one low-cost fund that tracks the S&P 500.

Is FSKAX a mutual fund or an ETF? FSKAX is a mutual fund, as is FXAIX. Both trade once a day at the closing price, while ETFs trade throughout the day on an exchange.

Is FXAIX the same as VOO? They track the same index, the S&P 500, so their returns are very close. FXAIX is a Fidelity mutual fund, while VOO is an ETF that can be bought at almost any broker.

Which is better for a Roth IRA, FXAIX or FSKAX? Either works well in a Roth IRA, because growth is never taxed there. The choice comes down to whether you want the largest companies or the whole market.

The Bottom Line

FXAIX and FSKAX cost the same and can both be moved to another broker, so the only real question is how much of the US market you want to own. In FRC’s data, the S&P 500 fund finished ahead in 10 of 14 years and turned $10,000 into about $4,070 more, but most of that lead came after 2019.

Neither approach wins in every period, and both have built strong long-term results. Learn to choose and combine funds like these with ETF Investing for Beginners, ETF Investing Strategies and Advanced ETF Investing.