A capitalization-weighted index is a stock market index in which each company’s influence on the index level is set by its market capitalization: share price multiplied by shares outstanding. The bigger the company, the bigger its pull on the number you see quoted. Most of the benchmarks investors actually watch, including the S&P 500, the Nasdaq Composite, and the FTSE 100, are built this way, which makes a cap-weighted index effectively a running scoreboard of where the most invested money sits.
How the Weighting Is Calculated
Start with market capitalization for each stock in the index: price times shares outstanding. Add every company’s market cap together to get the index’s total market value. Each company’s weight is its own market cap divided by that total. A company worth $200 billion inside a $2 trillion index carries a 10% weight; a $20 billion company carries 1%.
The published index level comes from dividing the sum of all constituent market caps by a number called the index divisor. For the S&P 500, the formula is Index Level = (sum of each stock’s price times its share count) divided by the Divisor.1S&P Global. S&P Dow Jones Indices Index Mathematics Methodology
A Three-Stock Example
Consider an index with only three stocks:
- Company A: 2 billion shares at $100 = $200 billion market cap
- Company B: 500 million shares at $80 = $40 billion market cap
- Company C: 1 billion shares at $10 = $10 billion market cap
Total market cap is $250 billion. Company A’s weight is $200B / $250B, or 80%. Company B gets 16%, Company C gets 4%. If Company A rises 5% the next day, that single move shifts the index far more than a 20% jump in Company C, because A carries twenty times C’s weight.
Why Not All Shares Count: Float Adjustment
Most major cap-weighted indices don’t use raw market cap. They use float-adjusted market cap, which counts only the shares that actually trade on the open market. Shares locked up by insiders, government entities, or corporate cross-holdings are stripped out because ordinary investors can’t buy them.
S&P Dow Jones Indices applies what it calls an Investable Weight Factor to each stock, equal to available float shares divided by total shares outstanding. Holdings are sorted into three categories: stakes held by other corporations, venture capital, or private equity firms; government holdings at any level; and shares held by officers, directors, founders, or company-controlled trusts and pension plans. If any category exceeds 10% of shares outstanding, that entire category’s holding is removed from the float count.2S&P Global. S&P Dow Jones Indices Float Adjustment Methodology
The effect is real. A company with a $100 billion market cap on paper counts as $60 billion in the index if a founding family holds 40% of shares. MSCI, which runs a separate family of global indices, defines free float in similar terms as the proportion of shares “available for purchase in the public equity markets by international investors.”3MSCI. Free-Float Adjustment in Global Equities – A Two-Decade Review
What the Divisor Does
The divisor keeps the index level continuous when the roster or share counts change. Without it, every stock split, buyback, or company swap would jolt the index for reasons that have nothing to do with actual stock prices.1S&P Global. S&P Dow Jones Indices Index Mathematics Methodology
When a company is added or removed, the provider recalculates the divisor after the close. The new divisor equals the old divisor multiplied by the ratio of the new total market value to the old one. The index opens the next morning at exactly the level it closed the day before, even though the underlying stocks have changed.1S&P Global. S&P Dow Jones Indices Index Mathematics Methodology The same fix is applied for stock splits, special dividends, rights offerings, and buybacks large enough to shift the count.
Why Large Companies Dominate
Cap-weighting isn’t subtle about who’s in charge. A stock representing 7% of the index moves the number about 70 times more than a stock representing 0.1% on the same percentage change. A handful of mega-caps end up setting the tone for the entire benchmark on most trading days.
That reflects reality in one sense. The biggest companies genuinely hold a huge share of total invested capital, and the index is designed to mirror that. It also means the index can hide what’s happening lower down. A broad benchmark can post a strong year driven almost entirely by five or six stocks while most of its constituents tread water or decline.
Concentration Risk and Momentum Bias
The same feature that makes cap-weighting intuitive creates its main vulnerability. Winners automatically gain index weight as their prices rise, and money flowing into index funds is then allocated proportionally to those largest holdings. Nothing in the methodology trims a stock that has grown expensive relative to its earnings.
This has burned investors before. Technology stocks swelled to roughly 34% of the S&P 500 by March 2000, right before the sector lost about 80% of its value. Financial stocks nearly tripled their index weight from 7% to 22% ahead of the 2008 crisis, then dropped 76%. In each case the structure kept adding to the sector about to implode.
Concentration today looks different but the dynamic is familiar. The top ten stocks in the S&P 500 account for roughly 37% of the index’s total weight, and most of those companies share exposure to artificial intelligence. At that level of concentration, an earnings miss at a single company can drag the whole index in a way that barely registered when the top holdings were spread across unrelated industries.
Major Cap-Weighted Indices
S&P 500
The S&P 500 tracks 500 large-cap U.S. companies selected by a committee at S&P Dow Jones Indices. Eligibility requires a total market capitalization of at least $22.7 billion, plus sufficient trading liquidity measured by a float-adjusted liquidity ratio. The committee also weighs sector balance against the broader U.S. equity market.4S&P Global. S&P U.S. Indices Methodology Rebalancing runs quarterly on the third Friday of March, June, September, and December.5CME Group. Navigating the S&P 500 Rebalance – A Quarterly Market Ritual
Nasdaq Composite
The Nasdaq Composite covers all common-type stocks listed on the Nasdaq Stock Market, both domestic and international, currently around 3,400 components. Because the Nasdaq exchange has historically drawn technology and growth companies, the composite tilts heavily toward those sectors.6Nasdaq Global Index Watch. Nasdaq Composite
FTSE 100
The FTSE 100 is the primary benchmark for the London Stock Exchange, made up of the 100 largest UK-listed companies by full market capitalization, screened for size and liquidity before float-adjusted weighting is applied.7FTSE Russell. FTSE UK Index Series Ground Rules The weighting design is identical in principle to the S&P 500, though eligibility rules and review cycles differ.8FTSE Russell. FTSE 100 Index
How You Actually Invest in One
You can’t buy an index directly. To get the exposure, you buy a fund, either an ETF or a mutual fund, built to replicate the index. The fund holds the same stocks in the same proportions and adjusts whenever the index rebalances. Fund returns always trail the index slightly, and that gap is called tracking difference.
The main driver is the expense ratio. A fund charging 0.03% annually should lag by roughly that amount, all else equal. A few other factors push the gap around:
- Cash drag: dividends collected from holdings sit briefly as cash before they’re reinvested.
- Transaction costs: buying and selling during rebalancing costs money, especially for indices with many, illiquid, or frequently changing constituents.
- Sampling: some funds skip the smallest holdings when the cost of owning them outweighs their tiny effect on returns.
- Securities lending: lending portfolio shares to short-sellers earns fees that can offset expenses and narrow the gap.
For large cap-weighted indices such as the S&P 500, these frictions are small because the underlying stocks trade heavily. Tracking gets harder with indices holding thousands of securities or covering less liquid markets.
Other Ways to Weight an Index
Cap-weighting is the default, not the only option. Knowing the alternatives clarifies what you’re getting.
A price-weighted index, like the Dow Jones Industrial Average, gives more influence to stocks with higher share prices regardless of company size. It’s the oldest design and the simplest to calculate, but the weighting is essentially arbitrary since a company can change its share price through a stock split without changing its underlying value.
An equal-weighted index assigns every stock the same weight at each rebalancing. The S&P 500 Equal Weight Index sets each of its 500 stocks to 0.20% every quarter.9S&P Dow Jones Indices. S&P 500 Equal Weight Index Methodology That eliminates mega-cap dominance and tilts toward smaller, more value-oriented companies, but requires constant rebalancing to pull weights back to parity, which generates more transaction costs and typically more volatility.
Fundamental-weighted indices set weights by financial metrics like revenue, cash flow, book value, and dividends rather than share price. The FTSE RAFI Index Series, for example, averages a company’s five-year sales, five-year cash flow, current book value, and five-year dividend history.10FTSE Russell. FTSE RAFI Index Series Methodology Overview Anchoring weights to business results rather than market sentiment avoids the momentum tilt built into cap-weighting, though the approach can lag during long stretches when expensive growth stocks keep winning.
No single method is objectively best. Cap-weighting offers the lowest maintenance costs and the widest fund availability. Equal-weighting spreads exposure at the cost of higher turnover. Fundamental weighting introduces a value discipline but relies on backward-looking financials. The right choice depends on whether concentration risk or tracking cost worries you more.