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Short answer: FXAIX holds the S&P 500 (about 500 large US companies); FSKAX holds the total US stock market (3,791 holdings, including mid and small companies). Both cost 0.015%. Over the last 10 years FXAIX returned +15.32% annualized vs +14.69% for FSKAX because large companies outperformed, but both are heavily weighted to the same mega-cap stocks.
FXAIX vs FSKAX at a glance
| FXAIX | FSKAX | |
|---|---|---|
| Index | S&P 500 | Dow Jones U.S. Total Stock Market Index |
| Expense ratio | 0.015% | 0.015% |
| Holdings (09/30/2026) | 508 | 3,791 |
| Inception | 02/17/1988 | 11/05/1997 |
| Turnover | 3% | 3% |
| Top 10 holdings | NVIDIA, Apple, Microsoft, Amazon, Alphabet, Broadcom, Meta, Micron, Tesla | Same names, totaling 36.10% of net assets |
Performance (annualized, through 09/30/2026)
| 1 year | 3 years | 5 years | 10 years | |
|---|---|---|---|---|
| FXAIX | +15.72% | +22.87% | +13.78% | +15.32% |
| FSKAX | +15.28% | +22.35% | +12.59% | +14.69% |
The gap is largest over 5 years (1.19 points a year). It comes from the mid and small companies FSKAX holds, which lagged large caps in this period. That can reverse. For comparison, Fidelity’s Extended Market Index Fund (FSMAX), which holds the part of the market the S&P 500 leaves out, returned +11.64% over 1 year, +6.17% over 5 years and +11.17% over 10 years.
How much do they overlap?
A lot. The same mega-cap companies sit at the top of both funds. Owning both adds little diversification, because FXAIX is essentially a large slice of FSKAX. If you already own FSKAX, adding FXAIX mostly increases your exposure to large companies.
Which should you choose?
- You want one fund for the whole US market: FSKAX.
- You want large US companies only or the exact S&P 500: FXAIX.
- You want to own both: you do not need both. Pick one, or pair a US fund with international and bond funds for real diversification.
Frequently asked questions
Is FSKAX better than FXAIX?
Neither is better in every period. FXAIX has outperformed in the last 1, 3, 5 and 10 years; FSKAX adds exposure to mid and small companies.
Should I own both FXAIX and FSKAX?
Usually not. They overlap heavily and cost the same.
Which has lower fees?
They are tied at 0.015%.
Related guides
Sources: Fidelity 500 Index Fund fact sheet (9/30/2026); Fidelity Total Market Index Fund fact sheet (9/30/2026); Fidelity Extended Market Index Fund fact sheet (9/30/2026).