FXAIX vs FSKAX (2026): Fidelity S&P 500 vs Total Market — Which Should You Own?
FXAIX (Fidelity 500 Index Fund) and FSKAX (Fidelity Total Market Index Fund) are two of Fidelity's flagship index funds — both completely free at 0.00%, both with no minimum investment, both holding the same mega-cap anchors. The difference is scope: FXAIX tracks 500 carefully selected large-cap companies; FSKAX tracks the entire US market, adding thousands of mid and small caps. We break down what that difference means for returns and your portfolio.
By Dan Mahler · Updated May 22, 2026
💡 Bottom Line Up Front
FXAIX has outperformed FSKAX across every time period tracked by CMF — 1-year (20.63% vs 20.30%), 5-year (13.08% vs 11.75%), and 10-year (15.05% vs 14.53% annualized). Both cost 0.00%. The gap is real but narrow, driven by large-cap tech dominance in the past decade. For most investors, either fund is an excellent core US equity holding. Choose FXAIX for the proven S&P 500 index; choose FSKAX if you want broader US market exposure and believe small and mid caps will outperform over your time horizon.
FXAIX vs FSKAX: At a Glance
| Metric | FXAIX | FSKAX |
|---|---|---|
| Fund Name | Fidelity 500 Index Fund | Fidelity Total Market Index Fund |
| Index Tracked | S&P 500 (~500 stocks) | DJ US Total Market (~2,500+ stocks) |
| Expense Ratio | 0.00% | 0.00% |
| Annual Cost on $100,000 | $0/year | $0/year |
| Minimum Investment | $0 | $0 |
| 1-Year Return | +20.63% | +20.30% |
| 3-Year Annualized Return | +20.05% | +19.55% |
| 5-Year Annualized Return | +13.08% | +11.75% |
| 10-Year Annualized Return | +15.05% | +14.53% |
| Distribution Yield | 1.09% | 0.99% |
| Volatility (Std Dev) | 12.14% | 12.52% |
| Total Assets (AUM) | $791.7B | $131.7B |
| Category | Large Blend | Total Market |
| Inception Date | May 2011 | September 2011 |
| Fund Family | Fidelity | Fidelity |
The Core Question: S&P 500 or the Whole US Market?
The S&P 500 is not the US stock market — it's approximately 80% of it by market capitalization. The remaining 20% is made up of mid-cap and small-cap companies: profitable businesses like regional banks, healthcare providers, industrial firms, and consumer brands that aren't large enough to meet S&P 500 inclusion criteria.
FXAIX gives you the 500. FSKAX gives you the full picture: those same 500 companies plus thousands more. Because the S&P 500 companies account for such a large share of total market cap, the top holdings in both funds look nearly identical — Apple, Microsoft, Nvidia, Amazon, Alphabet, Meta. The difference shows up further down the list.
In practice, this means FXAIX and FSKAX move together closely. Their correlation is extremely high — historically above 0.99. The performance gap between them widens or narrows based on whether large caps are leading or lagging. In the past decade, large caps won. That may not hold forever.
Performance Comparison: FXAIX Leads Across All Periods
Over every time period tracked by CMF, FXAIX has outperformed FSKAX. The gap is consistent but narrow:
| Time Period | FXAIX | FSKAX | FXAIX Edge |
|---|---|---|---|
| 1-Year Return | +20.63% | +20.30% | +0.33% |
| 3-Year Annualized | +20.05% | +19.55% | +0.50% |
| 5-Year Annualized | +13.08% | +11.75% | +1.33% |
| 10-Year Annualized | +15.05% | +14.53% | +0.52% |
Data from CMF fund database, updated May 2026. Past performance does not guarantee future results.
What $100,000 Grows To (Using Historical Annualized Returns)
| Holding Period | FXAIX | FSKAX | FXAIX Advantage |
|---|---|---|---|
| 5 Years (13.08% vs 11.75%) | ~$184,800 | ~$175,100 | +$9,700 |
| 10 Years (15.05% vs 14.53%) | ~$406,200 | ~$386,700 | +$19,500 |
These projections use historical annualized returns and do not account for taxes or future performance. Illustrative only.
The 10-year gap of roughly $19,500 on a $100,000 investment is meaningful — but it's important to understand its source. It reflects large-cap technology dominance, not a structural advantage of the S&P 500 over the total market. In periods where small and mid caps outperform — which they have historically over very long cycles — FSKAX would be expected to close that gap.
The Fee Gap: Both Are Free
Unlike most fund comparisons, cost is not a factor here. Both FXAIX and FSKAX charge 0.00%. Fidelity eliminated expense ratios on these funds in 2018 as part of its zero-fee index fund lineup. You pay nothing to own either fund — no annual fee, no transaction cost, no minimum investment.
This is a genuine differentiator compared to Vanguard, where the equivalent funds (VFIAX and VTSAX) charge 0.04% each and require $3,000 minimums. For investors at Fidelity, FXAIX and FSKAX are the most cost-efficient US equity index options available anywhere.
Because the fees are identical at 0.00%, the decision between FXAIX and FSKAX comes down entirely to index scope, not cost. Favor the data: returns, diversification, and your view on large-cap vs. total market going forward.
What Each Fund Owns: Inside FXAIX and FSKAX
FXAIX — What It Owns
- 📊 Tracks S&P 500 — 500 large-cap US stocks selected by committee
- 🏢 Only companies meeting strict size, liquidity, and profitability criteria
- 📈 ~80% of total US stock market cap in 500 names
- 🔄 No mid caps, no small caps — large-cap only
- 💵 $791.7B AUM — one of the largest mutual funds in the world
- 💰 0.00% expense ratio, $0 minimum
- 📊 12.14% historical volatility (slightly lower)
FSKAX — What It Owns
- 📊 Tracks Dow Jones US Total Market Index — ~2,500–5,000 US stocks
- 🏢 Large caps + mid caps + small caps — the full US market
- 📈 ~100% of investable US stock market cap
- 🔄 Same mega-caps at top, but diluted by smaller companies
- 💵 $131.7B AUM — significantly smaller than FXAIX
- 💰 0.00% expense ratio, $0 minimum
- 📊 12.52% historical volatility (slightly higher)
Despite the difference in index scope, the top 10 holdings of both funds look nearly identical: NVIDIA, Apple, Microsoft, Amazon, Alphabet, Broadcom, Meta, Tesla, Berkshire Hathaway. These mega-cap companies account for such a large percentage of total US market cap that they dominate both funds. The divergence happens in positions 50 through 2,500+.
The S&P 500 requires committee approval for inclusion — a company must be profitable on a GAAP basis for the most recent quarter and trailing four quarters. FSKAX's index includes companies that haven't yet met those criteria. In theory, this means FSKAX holds more earlier-stage companies with higher growth potential — and higher failure rates.
Risk: Nearly Identical, FXAIX Slightly Smoother
FXAIX has a standard deviation of 12.14% versus FSKAX's 12.52% — a difference of less than 0.4 percentage points. In practice, both funds move in tandem with the US stock market. The extra small and mid caps in FSKAX add marginally more volatility because smaller companies tend to be more sensitive to economic cycles than the large, diversified corporations in the S&P 500.
In bear markets — 2008, 2020, 2022 — small and mid caps typically fall more sharply than large caps, which means FSKAX would be expected to decline further than FXAIX in a downturn. The difference is small, but worth knowing for investors with shorter time horizons or lower risk tolerance.
Distribution yield slightly favors FXAIX (1.09% vs FSKAX's 0.99%). The gap is small, but income-focused investors may have a modest preference for FXAIX on that basis alone.
The Small-Cap Question: Does Broader Diversification Pay Off?
Financial theory argues that small-cap stocks carry a “size premium” — over very long horizons, they should outperform large caps because they carry more risk. The academic evidence for this premium is real but uneven: it has appeared and disappeared across different decades.
In the 2010s and early 2020s, large-cap tech companies dominated. Small and mid caps lagged, which is why FSKAX has trailed FXAIX. In the 1970s, 1980s, and early 2000s, smaller companies often led. There is no guarantee that large-cap dominance will persist over your investment horizon.
This is the honest case for FSKAX: if small and mid caps revert to historical averages or begin a leadership cycle, FSKAX is better positioned to capture that upside than FXAIX. FXAIX investors are betting — implicitly — that S&P 500 companies will continue to dominate US equity returns. It's a reasonable bet based on recent history, but not a certain one.
FXAIX vs FSKAX: Which Should You Choose?
Choose FXAIX if you…
- ✅ Want the proven S&P 500 index with a 90+ year history
- ✅ Already hold small or mid cap exposure elsewhere in your portfolio
- ✅ Prefer slightly lower volatility (12.14% vs 12.52%)
- ✅ Want marginally higher dividend yield (1.09% vs 0.99%)
- ✅ Prefer the most liquid, widely benchmarked US index fund
- ✅ Are using this as a core holding alongside international funds
Choose FSKAX if you…
- ✅ Want total US market exposure in a single fund
- ✅ Believe in the small-cap/mid-cap size premium over long horizons
- ✅ Don't want to miss a potential small-cap leadership cycle
- ✅ Prefer “own the whole market” philosophy over index selection
- ✅ Are building a simple two-fund or three-fund portfolio
- ✅ Have a 20+ year time horizon where small-cap premium may materialize
Our Verdict
FXAIX has the stronger recent track record and slightly lower volatility. FSKAX offers broader diversification that may pay off if smaller companies reclaim market leadership. Because both are completely free, this is not a cost question — it's a conviction question. Most investors holding either fund for 10+ years will do well. If you want to own the entire US market in one fund and not think about it, FSKAX is the purist choice. If you want the index that has historically won and is most widely tracked by financial benchmarks, FXAIX is the answer.
Frequently Asked Questions
Is FXAIX or FSKAX a better fund?
FXAIX has outperformed FSKAX across all time periods tracked by CMF: 1-year (20.63% vs 20.30%), 5-year (13.08% vs 11.75%), and 10-year (15.05% vs 14.53% annualized). Both cost 0.00%. FXAIX's edge reflects large-cap tech dominance in recent years. Over longer cycles, FSKAX's broader small and mid cap exposure may add return. For most investors, both are excellent core holdings.
What is the difference between FXAIX and FSKAX?
FXAIX tracks the S&P 500 (500 large-cap US companies, ~80% of US market cap). FSKAX tracks the Dow Jones US Total Market Index (~2,500–5,000 US companies including mid and small caps, ~100% of US market cap). Both charge 0.00% and require no minimum investment. The top holdings are nearly identical — the difference is FSKAX adds thousands of smaller companies below the S&P 500 threshold.
What is the expense ratio for FXAIX and FSKAX?
Both FXAIX and FSKAX have an expense ratio of 0.00%. Fidelity launched zero-expense-ratio index funds in 2018. There is no cost difference between these two funds — the choice is purely about index scope.
Should I own FXAIX or FSKAX in my 401(k)?
Both are excellent 401(k) core holdings. FXAIX gives you the S&P 500 — the most widely benchmarked US index. FSKAX gives you the full US market, preferred by investors following a two-fund or three-fund portfolio strategy. If your plan offers both, the choice depends on whether you hold other US equity funds (in which case FXAIX avoids small-cap duplication) or want a single all-US fund (FSKAX).
Why has FXAIX outperformed FSKAX?
FXAIX has outperformed because mega-cap technology companies (Apple, Microsoft, Nvidia, Amazon) have driven outsized US equity returns over the past decade. FSKAX includes these same companies but dilutes their weight by adding thousands of smaller stocks that have generally returned less. If small and mid caps enter a period of outperformance — as they have historically during economic recoveries and rising rate environments — FSKAX would be expected to close or reverse the gap.
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