Total-Market Funds and S&P 500 Funds: The Actual Difference

Someone choosing their first index fund almost always ends up in front of these two options, and the descriptions do not help. One tracks a few hundred large US companies. The other tracks the whole US market — thousands of companies. That sounds like a decisive difference.

It is a real difference, but a smaller one than the company counts imply, and the reason is worth understanding because it applies to nearly every fund comparison you will ever make.

Why the company count misleads

Both funds are weighted by company size. Each holding takes up space in the fund in proportion to what the company is worth, not in equal shares.

So the thousands of extra companies in a total-market fund are, by definition, the smaller ones — and being smaller, they take up correspondingly little of the fund. The large companies that make up the entire S&P 500 fund also make up the large majority of the total-market fund.

You are not comparing 500 companies against 3,500. You are comparing a fund made of large companies against a fund made of the same large companies plus a modest slice of smaller ones. That slice is the whole difference.

This is the same mechanism explored in how many funds a portfolio actually needs — size-weighting is why holding both of these funds together adds far less than it appears to.

What the difference actually gets you

A genuine, if modest, exposure to smaller US companies. Small companies do not move in lockstep with large ones; there are stretches where they do better and stretches where they do considerably worse. Owning them means your outcome depends slightly less on how a handful of very large companies happen to perform.

Modest is the honest word. Because of the weighting, the smaller companies cannot move the total much — that is precisely what "small" means in a size-weighted fund.

The comparison that usually matters more

For two funds this structurally similar, the deciding factor is often not which index they track. It is what each one costs.

An expense ratio is charged annually on the entire balance, so it compounds against you the way returns compound for you. Two funds holding nearly the same companies, separated by a fraction of a percent in cost, can end a working lifetime meaningfully apart. You can see the size of that gap in the fund comparison calculator — set both returns identical and move only the cost sliders.

There is also an asymmetry worth sitting with. Future returns are unknowable; whatever number you assume is a guess. The expense ratio is published, contractual, and known before you invest. One of those you can control.

How to actually choose

1. Decide whether you want exposure to smaller companies. Both answers are defensible; it is a preference, not a correct answer.
2. Compare the expense ratios of the specific funds available to you — in your workplace plan, the options may be limited and the costs may differ more than the indexes do.
3. Check what else you hold. If a total-market fund is going in beside an S&P 500 fund you already own, the second one is mostly buying what you have.
4. Then stop. This decision matters far less than whether you keep contributing, and people routinely spend months on it while contributing nothing.

The honest summary

For a single, long-term, broad US stock holding, either of these is a reasonable choice, and the difference between them is smaller than the time most people spend deciding. The costs, and your consistency, will shape the outcome more than the index will.

Look up the actual expense ratio and holdings of any fund in its own prospectus before deciding. Fund details change, and that document is the only authoritative version.