The total market value of a company's outstanding shares, calculated as share price multiplied by total shares outstanding.
Market capitalisation (market cap) is the most basic measure of a company's size as perceived by the market. Unlike book value (which reflects accounting values), market cap reflects what all investors collectively believe the company is worth today. The S&P 500 is a market-cap-weighted index: the largest companies by market cap receive the greatest weight and have the largest impact on the index's performance. Apple, Microsoft, and Nvidia have each at various times exceeded USD 3 trillion in market cap - each larger than many national economies.
Companies are broadly categorised by market cap: mega-cap (over USD 200 billion), large-cap (USD 10–200 billion), mid-cap (USD 2–10 billion), small-cap (USD 300 million–2 billion), and micro-cap (under USD 300 million). Smaller companies generally offer higher growth potential but greater volatility, thinner liquidity, and less analyst coverage. Large-caps tend to move more moderately but attract institutional capital flows that create more predictable technical behaviour.
For index traders (S&P 500 CFDs, ETFs), understanding market cap weighting explains why a small number of mega-cap stocks account for a disproportionate share of index performance. In 2023–2024, the 'Magnificent Seven' tech mega-caps contributed the majority of S&P 500 returns while the equal-weighted index significantly underperformed - a divergence that traditional P/E analysis would not have predicted.
Worked Example
Company A: 500 million shares outstanding, current price USD 80. Market cap = 500M × USD 80 = USD 40 billion (large-cap). If price rises 10% to USD 88, market cap rises to USD 44 billion. For a market-cap-weighted index holding this stock at 2% weight, the 10% stock gain adds 0.2% to the index return.