You've probably heard that the S&P 500 is "the market." Everyone talks about it like it's the only game in town. But honestly, if you only own the S&P 500, you are missing out on thousands of companies. This is where the U.S. Completion Total Stock Market Index comes in. It’s basically the "everything else" index.
Think of the U.S. stock market as a massive jigsaw puzzle. The S&P 500 is that big, easy-to-see section in the middle—the blue sky or the main house. It covers about 80% of the market value. But the other 20%? That’s the completion index. It’s the mid-caps, the small-caps, and the micro-caps that didn't make the cut for the big club.
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What is the U.S. Completion Total Stock Market Index anyway?
The Dow Jones U.S. Completion Total Stock Market Index is a mouthful, but its job is simple. It tracks every liquid, publicly traded U.S. company that is not in the S&P 500.
Most people don't realize that the S&P 500 isn't just "the 500 biggest companies." It’s a committee-selected list. To get in, a company has to be profitable and meet specific liquidity requirements. There are plenty of huge companies that aren't in it yet. Ever heard of Snowflake or Cloudflare? For a long time, they lived in the completion index.
As of early 2026, the index contains roughly 3,375 constituents. Compare that to the S&P 500, which... well, usually has about 500. By holding a fund that tracks this index, you're essentially "completing" your portfolio to cover the entire U.S. market.
The "Hidden" Growth Engine
Why would you want the leftovers?
Because today's mid-cap is tomorrow's giant. When a company is in the completion index, it's often in its high-growth phase. Once it gets "promoted" to the S&P 500, a lot of that initial explosive growth has already happened.
Take a look at the sectors. While the S&P 500 is heavily dominated by "The Magnificent Seven" and big tech, the U.S. Completion Total Stock Market Index is a bit more varied.
- Technology: Still big, but features names like Marvell Technology or Roblox.
- Industrials: You’ll find companies like Vertiv Holdings here.
- Health Care: Lots of biotech and mid-sized pharma like Alnylam Pharmaceuticals.
It’s a different vibe. It’s more volatile, sure. Small companies can swing 5% in a day because someone sneezed. But over long periods, that volatility can lead to higher returns.
S&P 500 vs. The Completion Index: The Real Performance Gap
Kinda weirdly, the S&P 500 has actually outperformed the broader market for much of the last decade. Usually, small-cap stocks are supposed to win over 20-30 years. That’s the "small-cap premium" academics talk about.
But recently? Large-cap tech has been an absolute juggernaut. In 2025, the S&P 500 returned about 17.9%. Meanwhile, many small-cap benchmarks were stuck in the single digits.
Does that mean the completion index is a bad bet? Not necessarily.
Markets move in cycles. There were long stretches—like the early 2000s—where the S&P 500 did nothing while the "extended market" (another name for completion stocks) went on a tear. If you only own the 500 giants, you're betting that the current trend of "big gets bigger" will last forever.
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How to actually use it in your portfolio
Most people use this index to "fix" their 401(k).
A lot of employer plans only offer an S&P 500 fund. If you just put all your money there, you’re missing 20% of the U.S. economy. You’ve basically ignored 3,000+ companies.
To build a DIY "Total Stock Market" fund, the math is pretty easy. You generally want a 4:1 ratio.
- 80% S&P 500 index fund
- 20% U.S. Completion Total Stock Market Index fund
This gives you a portfolio that looks almost exactly like the Vanguard Total Stock Market ETF (VTI) or the Schwab Total Stock Market Index Fund (SWTSX).
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Common Funds Tracking This Index
If you're looking to buy into this, you're usually looking for "Extended Market" funds.
- Vanguard Extended Market ETF (VXF): This is the big one. Very low fees (around 0.05%).
- Fidelity Extended Market Index Fund (FSMAX): Popular in 401(k) plans.
- iShares S&P Mid-Cap 400 or Russell 2500: These aren't exact matches but they cover similar ground. Just be careful with the Russell 2500—it actually has some overlap with the S&P 500, which can get messy.
The Risk Nobody Mentions
Small companies are fragile.
When interest rates stay high, small companies feel the burn first. They often have more debt and less "moat" than Apple or Microsoft. If the economy hits a wall, the U.S. Completion Total Stock Market Index usually drops harder and faster than the S&P 500.
In December 2025, for example, market breadth weakened. While the big guys held steady, the advance/decline ratio for smaller stocks tanked. More stocks were falling than rising. That’s the price you pay for the potential "moonshot" returns of a future tech giant.
Actionable Steps for Your Money
If you want to move beyond just the S&P 500, don't just dump everything into a completion fund tomorrow. That’s a recipe for a heart attack when the market dips.
- Check your current overlap: Use a tool like Morningstar’s "Instant X-Ray" to see how much mid and small-cap exposure you already have. You might be surprised.
- Balance the 80/20 split: If your 401(k) is 100% S&P 500, consider moving 15-20% into an "Extended Market" or "Completion" fund to capture the full U.S. economy.
- Watch the fees: Don't pay more than 0.10% in expense ratios for these funds. Since they are passive indexes, there's no reason to pay a premium for a "manager" who is just following a list.
- Rebalance annually: Small-caps can have a "good year" and suddenly take up 30% of your portfolio. Sell some of the winners and move them back into the S&P 500 to keep your risk in check.
Diversification isn't about picking the "best" index; it's about making sure you aren't standing on a one-legged stool. The S&P 500 is a great leg, but adding the completion index gives the whole thing a lot more stability over the long haul.