Index Funds Explained: The Flavors, The Numbers, And Which One To Actually Buy

TL;DR

• An index fund owns everything on a list (an index) instead of paying a human to guess. It wins because it's nearly free and never has a bad stock-picking year. It just is the market.
• The main flavors: S&P 500 (500 big US companies), total US market (~3,500 companies), global (~9,000 companies worldwide), plus narrower cuts like Nasdaq-100 and equal weight.
• The performance difference between S&P 500 and total US market has been nearly zero for decades. The fee difference between good funds is also nearly zero. Don't agonize.
• "Best" mostly means broad + cheap + tax-sheltered + left alone. Everything else is detail.

Already know what an index fund is? Skip to "The Flavors" below, which is where the numbers live.

What An Index Fund Is (One Minute Version)

An index is just a list of companies with a rule attached. The S&P 500 is "the 500 or so biggest US public companies, weighted by size." An index fund is a fund that buys that entire list, in those proportions, automatically. No manager making calls, no research team, no genius required, which is precisely why it's cheap, and precisely why it works.

The case for it is one statistic: over 15-year periods, roughly 90% of actively managed funds underperform their index after fees. You're not settling for average by buying the index. You're locking in a result that beats almost every professional trying to do better. The market's long-run average return has been about 10% a year before inflation (call it ~7% after), including every crash along the way.

One vocabulary note: most index funds today come as ETFs (trade like a stock, buy through any broker) or mutual funds (buy from the fund company, price set once daily). For a long-term buyer the difference is plumbing. Buy whichever your account makes cheap and automatic.

The Flavors

S&P 500, the default. 500-ish large US companies, size-weighted. Cheapest tickets: VOO (0.03%), IVV (0.03%), Fidelity's FXAIX (0.015%). Note that SPY charges 0.09%, three times VOO for the same index. SPY's advantage is liquidity in its options market, which matters to active traders and is worth nothing to a buy-and-hold investor paying the difference for decades. One thing to know: size-weighting means the top 10 companies are roughly a third of the fund, so you're more concentrated in tech giants than "500 companies" sounds.

Total US market, the S&P 500 plus everything smaller. Around 3,500 companies, but still size-weighted, so the small companies are a rounding error: total-market funds and S&P 500 funds have tracked each other within a whisker for decades, and long-run annual returns differ by roughly 0.1%. Tickets: VTI (0.03%), FSKAX (0.015%), and Fidelity's FZROX (0.00%, literally free). Pick this or the S&P 500; flipping a coin is a defensible methodology.

Global, the whole world in one fund. ~9,000 companies, roughly 60% US / 40% everywhere else. Tickets: VT (0.07%), or a DIY pair of VTI + VXUS (0.05%). The pitch for global: US stocks have crushed international for the last 15 years, which is exactly why nobody wants global funds, and exactly the kind of streak that has reversed before (international won the 2000s; the US did nothing for that decade). Global is the "I refuse to bet on which country wins" option, and that's a respectable refusal. It's also a reminder that home bias cuts both ways: UK investors watched the FTSE 100 go sideways for years while the S&P compounded.

Nasdaq-100, the tech-heavy one. The 100 largest Nasdaq-listed companies, which in practice means a concentrated bet on big tech. Tickets: QQQ (0.20%) or its cheaper twin QQQM (0.15%). Spectacular last 15 years; also fell ~80% in 2000–2002 and took 15 years to reclaim its dot-com high. This is a sector tilt wearing an index costume: fine as a side dish, dangerous as the whole meal.

Equal weight, every company gets the same slice. RSP (0.20%) holds the S&P 500 but puts 0.2% in each company, so you're not a third invested in the top 10. More diversified across companies, tilted toward smaller ones, higher fee, and it trades more (which can mean more tax drag outside a shelter). A reasonable answer to "isn't the S&P too top-heavy?", at nearly 7x the fee of VOO.

Dividend index funds get their own guide, Dividend Stocks & Funds: Pros, Cons and the Free-Lunch Illusion, because the appeal and the catch both deserve space.

A note for readers outside the US. Every ticker above is a US-domiciled fund, and that is a fact about availability, not a recommendation. UK and EU brokers generally cannot sell US-domiciled ETFs to retail clients at all, because those funds do not publish the disclosure document European rules require, so the practical choice there is a UCITS equivalent tracking the same index under a different ticker. Domicile also drives withholding tax on dividends and how the holding is taxed in your own country. Fund availability, tax treatment and the right wrapper all depend on where you live, so check what your broker can actually offer you before comparing fees.

So Which Is "Best"?

By the numbers, any broad one with a fee at or under ~0.1%, held for decades. The gap between VOO, VTI, FXAIX, and FZROX is noise, a few dollars a year per $10,000. The genuinely consequential choices are only these:

• US-only vs global. The one real philosophical fork. US-only is a bet America keeps winning; global is declining to make that bet for ~0.04% more in fees.
• Broad vs narrow. S&P 500 / total market / global are complete meals. Nasdaq-100, sector funds, and themed ETFs ("AI & Robotics," 0.68%!) are concentrated bets with index branding, and the narrower the theme and the higher the fee, the more it's marketing.
• Cheap vs not. Same index at 0.03% vs 0.9% is the same product with a 30x markup. Why fees matter this much: Fund Fees Explained.

What's not on the list: leveraged index funds. "The S&P but 3x" sounds like the obvious upgrade, and it reliably isn't. The math is genuinely surprising and gets its own guide: Why Not Just Buy A Leveraged S&P Fund?

The Part Everyone Skips

Whichever you pick, the fund is maybe 10% of the outcome. The other 90% is behavior: buying automatically every month (the how and why), and not selling in the crash years. 2008 (−37%), 2020, 2022 (−19%) were all part of that 10% long-run average. The index fund's real superpower isn't the diversification or even the fees. It's that owning everything gives you nothing in particular to panic about.