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 — that's 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; it exists for traders, not for you. 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 — 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 honest pitch: 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: The Honest Pros and Cons — because the appeal and the catch both deserve space.

So Which Is "Best"?

By the numbers, the honest answer is boring: 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 — 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.