Index Overlap: S&P 500 and Total Market Funds

The S&P 500 comprises roughly 500 large-cap stocks and captures approximately 80–85% of the total US stock market’s capitalization, leaving 15–20% in mid-cap and small-cap stocks. An investor holding both an S&P 500 fund and a total market fund is redundant in that 80–85% overlap but gains exposure to smaller companies in the remainder—a trade-off between simplicity and marginal diversification.

Market cap concentration

The US stock market contains roughly 3,500–4,000 publicly traded companies (including all exchanges). The S&P 500 is a curated subset: the 500 largest by market capitalization, selected by Standard & Poor’s index committee for liquidity and financial health.

These 500 companies represent ~80–85% of total US equity market capitalization in any given year. The remaining 15–20% is scattered across:

• Mid-cap stocks (market cap ~$2–$10 billion): ~600–700 names, ~8–10% of market value.
• Small-cap stocks (market cap ~$300 million–$2 billion): ~2,000+ names, ~5–7% of market value.
• Micro-cap stocks (below $300 million): ~1,000+ illiquid names, ~1–2% of market value.

This concentration is not fixed. In bull markets for small stocks, the S&P 500’s share of total value shrinks to 75–78%. In strong large-cap runs (like the 2015–2022 “Magnificent 7” tech rally), it expands to 85–87%.

Why the overlap matters

An investor seeking broad US market exposure has two common choices:

1. Buy an S&P 500 fund (captures 80–85% of the market by value).
2. Buy a total US stock market fund (captures 100%).

A total market index includes the S&P 500 companies plus everything else. If you buy both simultaneously, you’ve overweighted the 500 large-cap stocks by roughly 60–80% (depending on the weighting in the total market fund) and added exposure to mid- and small-cap names.

The arithmetic is intuitive: 80% overlap means 20% unique. If you allocate $1,000 to S&P 500 and $1,000 to total market, you’ve effectively allocated $1,600 to the S&P 500 names (80% of the total market fund), $400 to smaller names (the remainder), and duplicated $800 in the large-cap tier.

Empirical performance gap

Historically, the long-term returns of an S&P 500 fund and a total US market fund are very similar—within 0.2–0.5% annualized over 20+ year periods.

But the gap widens in specific cycles:

When large-cap stocks are expensive relative to smaller peers, a total market fund pulling in small-cap exposure outperforms. When large-cap growth is hot (as in NASDAQ rallies), the S&P 500 pulls ahead. Over the full 40-year horizon (1980–2020), both delivered nearly identical returns.

The cost of redundancy

If you hold both S&P 500 and total market funds, you’re paying:

• Double fees: If the S&P 500 fund charges 0.03% and total market charges 0.04%, you’re paying ~0.035% on overlapping assets and 0.04% on the marginal 20%.
• Double trading costs: Rebalancing one fund requires trading; rebalancing both requires trading both, incurring higher spreads.
• Unnecessary tax drag: In taxable accounts, both funds rebalance independently, creating taxable events.

A rough annual cost: 0.06–0.10% in fees and trading on the overlap alone, or $6–$10 per $10,000 invested. Over 30 years at 7% annualized returns, that’s 1–2% of compounded value lost to redundancy.

When to hold both

Holding both can make sense in a few cases:

1. Systematic allocation plan: Some investors allocate 80% to S&P 500 and 20% to small-cap (via a total market fund’s small-cap subset). This is an intentional size tilt, not an accident.

2. Separate accounts or institutions: Pension funds may hold an S&P 500 index fund for the large-cap sleeve and a total return extension (a small-cap fund) separately, for accounting or governance reasons.

3. Transitioning portfolios: Moving from all S&P 500 to total market exposure over time to minimize trading impact.

For most individual investors, redundancy is a mistake. A single index-fund tracking the sp-500-index or total market is simpler and cheaper.

Alternative: Total market as the core

Many advisors recommend:

• Core: 100% of equity in a total US market index fund (one fund, ~0.04% fee, full diversification).
• Tactical tilt (optional): Add a small-cap or equal-weight fund if you hold a specific view on size premiums.

This approach:

• Owns all 3,500+ stocks in proportion to market value.
• Pays a single fee (lowest cost).
• Avoids redundancy and tracking error.
• Automatically rebalances by market cap (large-cap names grow, small-cap names shrink or vice versa, without manual rebalancing).

The sp-500-index captures the bulk of US equity returns and is sufficient for most investors. Total market adds 15–20% of names and a similar magnitude of return at minimal cost. The question is whether that marginal exposure justifies its cost—usually it does.

See also