The Formal Definition
The critical phrase is all investable. A total market index does not screen by company size, sector, profitability, or any factor beyond basic eligibility — minimum market capitalisation, liquidity, and exchange listing requirements. If a company is publicly traded and meets the minimum size thresholds, it belongs in the index.
This distinguishes total market indices from virtually every other well-known benchmark. The S&P 500 imposes size minimums and committee selection. Sector indices restrict by industry. Dividend indices require payment history. A total market index makes no such judgements. It holds everything.
How It Differs from Other Indices
Understanding the total market index is easier when compared to the alternatives retail investors most commonly encounter.
S&P 500
The S&P 500 covers the 500 largest US companies by market capitalisation, as selected by an S&P committee. These 500 companies represent roughly 80% of total US market cap. The index deliberately excludes mid-cap and small-cap stocks. By count, the S&P 500 holds about 12% of the approximately 4,000 companies in the US total market — but by dollar value, those 500 companies dominate.
A US total market index includes those same 500 large-cap companies — and then adds the remaining ~3,500 mid-cap and small-cap companies that the S&P 500 excludes.
Dow Jones Industrial Average
The DJIA tracks 30 large, well-established US companies, selected by the editors of The Wall Street Journal. It is price-weighted rather than market-cap weighted, meaning a $200 stock affects the index more than a $50 stock regardless of company size. With just 30 holdings and a non-market-cap weighting methodology, the Dow is widely considered an imprecise measure of the broader market — more a historical artifact than an analytical tool.
Sector Indices
Sector indices — such as the S&P Technology Select Sector or the S&P Financials Select Sector — cover only companies in a specific industry. They are useful for targeted sector exposure but by design represent a fraction of the overall market. A total market index holds all sectors simultaneously, proportioned by their actual weight in the economy.
Russell 2000
The Russell 2000 covers the 2,000 smallest companies in the Russell 3000 — a small-cap complement to large-cap indices. While useful for small-cap exposure, it is not a total market index. A total market index spans large, mid, and small caps in a single instrument.
| Index | Holdings | Weighting | Coverage of US Market |
|---|---|---|---|
| CRSP US Total Market | ~4,000 | Market cap | ~100% |
| S&P 500 | ~500 | Market cap | ~80% |
| Russell 2000 | 2,000 | Market cap | ~8–10% (small cap only) |
| Dow Jones Industrial Average | 30 | Price-weighted | <5% |
How Market-Cap Weighting Works
A market-cap weighted index assigns each component a weight proportional to its total market value relative to the entire index. If a company's market capitalisation is $3 trillion and the total market cap of all stocks in the index is $50 trillion, that company receives a 6% weight in the index.
The practical consequence: large companies dominate the returns of a total market index, just as they dominate the actual economy. The top 10 companies in the CRSP US Total Market Index collectively represent roughly 25–30% of its total value, even though they account for less than 0.25% of the stocks in the index.
This is not a flaw — it is an accurate reflection of economic reality. Large companies are worth more because they produce more value. Market-cap weighting ensures the index reflects the actual distribution of wealth in the publicly traded market.
Free-Float Adjustment
Most modern total market indices apply a free-float adjustment. Rather than weighting by total shares outstanding, they weight by free-float shares — only the shares actually available for purchase on the open market.
Shares held by founders, controlling families, governments, or corporate insiders subject to lock-up agreements are excluded from the float calculation. The rationale: an investor cannot buy shares that are not available for sale. The free-float adjustment makes the index more accurately reflect the investable universe.
For most large-cap US companies, the free float is close to 100% — most shares trade freely. The free-float adjustment has a larger effect on companies where insiders retain major stakes, such as recent IPOs or family-controlled businesses.
Why Total Market Coverage Matters
The diversification argument for total market investing is straightforward: if you own every stock in proportion to its market value, you cannot underperform "the market" by definition. You are the market.
More specifically, total market exposure provides three advantages over partial-market indices:
The Small-Cap Premium
Academic research — most notably the Fama-French three-factor model (1993) — documents a historical tendency for small-cap stocks to outperform large-cap stocks over long periods, adjusting for risk. This "small-cap premium" is not guaranteed, and the evidence in recent decades has been mixed. But a total market index captures whatever small-cap return exists automatically, without any active decision to tilt toward small caps.
Sector Concentration Risk
A total market index holds every sector — technology, healthcare, financials, consumer staples, energy, industrials, and all others — in proportion to their actual market weight. If technology stocks decline sharply, the impact on a total market index holder is cushioned by exposure to other sectors. An S&P 500 holder has the same broad exposure, but a sector-index holder bears concentrated risk.
No Benchmark Risk
If your goal is to match the aggregate return of the US equity market, holding a total market index ensures you will. You cannot underperform the market you're trying to match, because the index defines the market. Holding any partial index introduces the possibility of tracking below the true total market benchmark.
The Origins: John Bogle's Insight
The intellectual foundation for total market investing traces to John Bogle, founder of Vanguard. In the early 1970s, Bogle argued — drawing on academic research in efficient market hypothesis — that most active fund managers, after fees, fail to consistently outperform a simple market index over time.
His conclusion: give investors a fund that does nothing but hold the index, charge the lowest possible fees, and most investors will outperform the majority of actively managed alternatives over the long run. The first S&P 500 index fund launched in 1976. By 1992, Vanguard extended the concept to the total market with the Vanguard Total Stock Market Index Fund — the first retail fund giving individual investors access to every publicly traded US company in a single, low-cost vehicle.
Read the full history of total market investingWho Uses Total Market Indices
Retail Investors
The primary audience for total market index funds. Investors following passive or "Boglehead" strategies often hold a single total market fund as their entire US equity allocation — sometimes combined with an international total market fund and a bond index fund in a simple three-fund portfolio.
Institutional Investors
Pension funds, endowments, and foundations use total market indices as benchmarks against which to measure active managers. Some hold total market index funds directly as passive core holdings.
Financial Researchers
The total market index provides the cleanest available measure of aggregate equity market returns for academic research. Because it includes every eligible company, it avoids the selection biases present in partial-market indices.
Financial Advisors
Total market index funds are widely recommended by fee-only financial planners as a low-cost, broadly diversified core equity holding for clients with long investment horizons. Their low expense ratios — often below 0.05% — preserve more return for the investor than actively managed alternatives.