There is no single best S&P 500 index fund for a Roth IRA. The right pick is whichever tracks the index at the lowest cost inside the brokerage you already use. At Fidelity that is FXAIX (0.015%), at Schwab it is SWPPX (0.02%), and for a portable ETF, VOO or IVV (both 0.03%) work anywhere.
Key takeaways
- Inside a Roth IRA there is no tax on dividends or capital gains distributions, so the tax-efficiency edge ETFs hold in a taxable account disappears. Cost and platform fit decide the winner.
- Every fund below tracks the same S&P 500, so pick on expense ratio and convenience, not on hoped-for performance. A 500-stock index fund is a 500-stock index fund.
- Match the fund to your broker: FXAIX at Fidelity, SWPPX at Schwab, VFIAX or VOO at Vanguard. Buying a rival house fund can trigger a transaction fee that dwarfs the expense ratio gap.
- Mutual funds let you auto-invest a fixed dollar amount and buy fractional shares by dollar. ETFs trade intraday and move between brokers in kind without selling.
- FNILX carries a 0.00% expense ratio but does not track the S&P 500. It follows Fidelity's own U.S. Large Cap Index, so returns will drift from the benchmark. Treat it as an S&P 500 near-substitute, not a match.
The funds compared
| Fund | Ticker | Expense ratio | Type | Best home |
|---|---|---|---|---|
| Fidelity 500 Index Fund | FXAIX | 0.015% | Mutual fund | Fidelity |
| Schwab S&P 500 Index Fund | SWPPX | 0.02% | Mutual fund | Schwab |
| Vanguard S&P 500 ETF | VOO | 0.03% | ETF | Any broker |
| iShares Core S&P 500 ETF | IVV | 0.03% | ETF | Any broker |
| Vanguard 500 Index Admiral | VFIAX | 0.04% | Mutual fund | Vanguard |
| SPDR S&P 500 ETF Trust | SPY | 0.0945% | ETF | Traders |
| Fidelity ZERO Large Cap | FNILX | 0.00% | Mutual fund | Fidelity (not S&P 500) |
On a $100,000 balance, the spread from FXAIX at 0.015% to SPY at 0.0945% is about $80 a year. Real money over decades, but small enough that platform convenience usually matters more than shaving a basis point.
Why the Roth changes the calculus
In a taxable brokerage account, the mutual fund versus ETF question carries real weight. Mutual funds can pass through capital gains distributions that you owe tax on even if you never sold a share, while ETFs use in-kind creation and redemption to sidestep most of that. Inside a Roth IRA none of it applies. Growth is tax-free, qualified withdrawals are tax-free, and a year-end distribution changes nothing on your return.
That removes the main reason to prefer an ETF and hands the decision to two plain factors: which fund costs the least, and which one your brokerage lets you buy without friction. For a deeper look at the index itself, see our S&P 500 hub.
Pick the cheapest tracker at your own broker
The practical rule is simple. Open the fund your brokerage runs in-house and buy that.
- Fidelity account: FXAIX at 0.015% is the cheapest true S&P 500 fund on the market, with no minimum and fractional buying by dollar amount.
- Schwab account: SWPPX at 0.02%, no minimum, same auto-invest convenience.
- Vanguard account: VFIAX at 0.04%, or VOO at 0.03% if you want the ETF share class with no minimum beyond one share.
- Robinhood, M1, or any broker: an ETF like VOO or IVV, since they trade commission-free everywhere and are not tied to one fund family. If you are weighing where to hold the account, our Robinhood vs Fidelity Roth IRA comparison lays out the tradeoffs.
Buying another firm's mutual fund in your account can cost a flat transaction fee, sometimes $50 or more, which erases years of expense-ratio savings in a single trade. When in doubt, stay in the family or use an ETF.
Mutual fund or ETF inside the Roth
Both structures hold the same 500 companies. The differences are mechanical.
A mutual fund prices once a day after the close, lets you invest an exact dollar figure, and supports automatic recurring contributions down to the penny. That makes it the easy default for a set-and-forget Roth where you contribute every month.
An ETF trades throughout the day at a live price, and it transfers between brokers in kind, so you can move it to a new custodian without selling and rebooking. For most Roth investors buying once or twice a month and never touching it, that flexibility is nice to have rather than decisive.
The FNILX caveat
Fidelity's ZERO funds advertise a 0.00% expense ratio, and FNILX is real, with no gimmick on the fee. The catch is the benchmark. FNILX does not track the S&P 500. It tracks the Fidelity U.S. Large Cap Index, an in-house index Fidelity built partly to avoid paying S&P licensing fees. In practice the holdings look similar and returns run close, but they will not match the S&P 500 exactly, and the fund only lives inside Fidelity accounts. If you specifically want the S&P 500, use FXAIX. If you want zero cost and can accept a near-identical but distinct large-cap index, FNILX is defensible.
What about SPY
SPY is the oldest and most heavily traded S&P 500 ETF, which makes it the tool of choice for options and active traders who value tight spreads and deep liquidity. For a buy-and-hold Roth IRA, none of that helps you, and its 0.0945% expense ratio is roughly three times VOO or IVV. Skip it for retirement holding. For the fuller breakdown of the ticker versus the underlying index, see SPY vs the S&P 500, and if you are comparing Vanguard's mutual fund to the benchmark directly, Vanguard 500 Index Fund vs the S&P 500 covers it.
Bottom line
Stop hunting for the one perfect fund. Every option here tracks the same index, and inside a Roth the tax questions that separate mutual funds from ETFs fall away. Buy the lowest-cost S&P 500 tracker your brokerage offers, automate the contributions, and let tax-free compounding do the rest.
Frequently asked questions
What is the best S&P 500 index fund for a Roth IRA?
There is no single best one; the right pick is whichever tracks the index at the lowest cost inside the brokerage you already use. At Fidelity that is FXAIX (0.015%), at Schwab it is SWPPX (0.02%), and for a portable ETF, VOO or IVV (both 0.03%) work anywhere. Every option tracks the same 500 stocks.
Does mutual fund versus ETF matter inside a Roth IRA?
No, it matters far less inside a Roth. There is no tax on dividends or capital gains distributions in a Roth, so the tax-efficiency edge ETFs hold in a taxable account disappears. That hands the decision to two plain factors: which fund costs the least, and which one your brokerage lets you buy without friction.
Why should you buy your own broker's S&P 500 fund?
Because buying another firm's mutual fund can cost a flat transaction fee, sometimes $50 or more, which erases years of expense-ratio savings in a single trade. Open the fund your brokerage runs in-house: FXAIX at Fidelity, SWPPX at Schwab, VFIAX or VOO at Vanguard, or an ETF like VOO or IVV at Robinhood, M1, or any broker.
Is FNILX an S&P 500 fund?
No, FNILX carries a 0.00% expense ratio but does not track the S&P 500. It follows Fidelity's own U.S. Large Cap Index, an in-house benchmark built partly to avoid S&P licensing fees, so returns will drift from the S&P 500. Treat it as a near-substitute, not a match; if you specifically want the S&P 500, use FXAIX.
Should you hold SPY in a Roth IRA for the long term?
Probably not. SPY is the oldest and most heavily traded S&P 500 ETF, which suits options and active traders who value tight spreads and deep liquidity, but its 0.0945% expense ratio is roughly three times VOO or IVV. For a buy-and-hold retirement holding none of that helps you, so a cheaper tracker is the better choice.