You probably know the S&P 500 like the back of your hand. It’s the "market," right? Well, not exactly. If you only own the S&P 500, you’re essentially ignoring thousands of companies that keep the American economy humming. That’s where the Wilshire 4500 Completion Index comes in.
Most people I talk to think of the stock market as a monolithic block of tech giants. But honestly, if you want the full picture, you have to look at what’s left over when the giants leave the room. The Wilshire 4500 is basically the "everything else" index. It’s the "completion" because it completes the Wilshire 5000 (which tracks nearly every publicly traded US stock) by excluding the S&P 500.
What Is the Wilshire 4500 Completion Index Anyway?
Think of the Wilshire 5000 as the entire ocean of US stocks. Now, take a massive net and scoop out the 500 biggest fish—the Apples, the Microsofts, the Teslas. What’s left in the water is the Wilshire 4500 Completion Index.
Even though the name says "4500," the actual number of stocks in it varies. It’s not a static list. As of early 2026, it captures the mid-cap, small-cap, and micro-cap companies that the S&P 500 ignores. It’s weighted by float-adjusted market capitalization, meaning the bigger "small" companies have more influence than the tiny "micro" ones.
Why "Completion" is a weird but perfect name
The term "completion" exists because this index is designed to be paired. If you already have an S&P 500 fund—which most 4001(k) and IRA investors do—and you add a Wilshire 4500 fund, you’ve essentially "completed" your coverage of the total US stock market. You aren't doubling up on the same companies; you're filling in the gaps.
The Secret Sauce: Mid and Small-Cap Exposure
The Wilshire 4500 is where the growth stories usually start. While the S&P 500 is full of "mature" companies that have already peaked, the 4500 is packed with the innovators and the disruptors.
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You’ve got companies in there that are just on the verge of becoming household names. Historically, this index has been a favorite for the Federal Retirement Thrift Investment Board. For years, the Thrift Savings Plan (TSP) "S Fund" tracked the Wilshire 4500 to give government employees exposure to small and mid-sized companies. While some versions of these funds have shifted to similar benchmarks like the Dow Jones U.S. Completion Total Stock Market Index, the core philosophy remains the same: capture the "extended market."
It's a Wild Ride
Let’s be real: the Wilshire 4500 Completion Index is more volatile than the S&P 500. It's just the nature of the beast. Smaller companies don't have the massive cash cushions that a Meta or Alphabet has. When the economy gets shaky, these stocks tend to drop faster. But when the bulls run? They can outpace the giants significantly.
How It Differs from the Russell 2000
This is a common point of confusion. People ask, "Isn't the Russell 2000 the small-cap index?"
Yes, but it's different. The Russell 2000 is strictly small-cap. The Wilshire 4500 is broader. It includes mid-cap stocks that aren't quite big enough for the S&P 500 but are way too big for the Russell 2000.
- S&P 500: The 500 biggest.
- Russell 2000: Stocks ranked roughly 1,001 to 3,000 in size.
- Wilshire 4500: Everything in the Wilshire 5000 except the S&P 500.
Basically, the Wilshire 4500 gives you a much smoother transition between "tiny" and "huge." It doesn't leave out that crucial mid-cap segment that often hits the "sweet spot" of risk and reward.
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Real Talk on Performance and Risk
If you looked at the data in 2024 and 2025, you’d see a tug-of-war. The "Magnificent Seven" tech stocks often carried the S&P 500 to record highs, while the broader market—tracked by the Wilshire 4500—sometimes lagged behind.
But history is a long game. Diversification isn't about winning every year; it's about not losing everything when one sector fails. If tech takes a massive hit, the industrial, healthcare, and regional bank stocks that populate the Wilshire 4500 might be what keeps your portfolio from sinking.
Sector Breakdown
Surprisingly, the Wilshire 4500 Completion Index isn't just a bunch of tech startups. It's heavily weighted toward:
- Financials: Regional banks and insurance firms.
- Industrials: The companies that actually build things and move freight.
- Information Technology: Not the giants, but the software firms you've never heard of.
- Health Care: Biotech and specialized medical device companies.
How to Actually Invest in It
You can't buy an index directly—you need a fund that mimics it. For a long time, the go-to was the Vanguard Extended Market Index Fund (VEXAX). It doesn't track the Wilshire 4500 specifically anymore (it moved to the S&P Completion Index), but for most investors, the difference is negligible. They are essentially twins.
If you’re looking for a direct path, check your brokerage for "Extended Market" ETFs or Mutual Funds.
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- Check your current holdings. If 80% of your money is in an S&P 500 fund (like VOO or SPY), you are missing the extended market.
- The 80/20 Rule. A classic strategy is putting 80% into the S&P 500 and 20% into the Wilshire 4500. This roughly replicates the "Total Stock Market" return.
- Rebalance annually. Small caps can grow into large caps (meaning they leave the 4500 and join the 500). Large caps can shrink and fall back into the 4500. You need to keep your ratios in check.
What Most People Get Wrong
The biggest misconception is that the Wilshire 4500 is "safer" because it has more stocks. More stocks does equal more diversification, but it doesn't mean less risk.
Individual companies in the 4500 are more likely to go bankrupt than companies in the 500. However, the index as a whole is resilient because it’s a cross-section of the entire American entrepreneurial spirit.
Honestly, ignoring the Wilshire 4500 Completion Index is like trying to understand a book by only reading the chapter titles. You get the big ideas, but you miss all the character development and the plot twists that actually make the story work.
Actionable Next Steps for Your Portfolio
If you're ready to move beyond the "S&P 500 only" mindset, here is how to handle the Wilshire 4500 Completion Index right now:
- Audit your exposure: Look at your "style box" on Morningstar or your brokerage app. If your "Mid" and "Small" cap boxes are nearly empty, you have a gap that the Wilshire 4500 is designed to fill.
- Compare expense ratios: When looking for an "Extended Market" fund, don't pay more than 0.10% in fees. Since these are passive indexes, there is no reason to pay a premium for "active management."
- Use it for "Completion": Don't make this your entire portfolio. It’s a tool for balance. Pair it with your large-cap holdings to ensure you own a piece of every profitable (and some not-yet-profitable) company in America.
By incorporating the extended market, you aren't just betting on the winners of today—you're buying a ticket for the winners of tomorrow.