The simple calculation
If a company has 1 billion shares outstanding and each share trades at £50, its market cap is £50 billion. That’s it. The formula never changes: Market cap = share price × shares outstanding. Market cap is a snapshot — it changes every second markets are open as the share price moves. It is fundamentally different from a company’s book value (the accounting value of assets minus liabilities) and its enterprise value (which also accounts for debt and cash).
Why size matters in investing
Large-cap companies are generally more liquid, better analysed, and less volatile than small-caps. They dominate major indices and attract institutional money. Small-cap stocks are less well-covered by analysts, can be harder to buy and sell in size, and tend to be more volatile — but historically have delivered higher long-run returns than large-caps (the "small-cap premium," though it has been inconsistent in recent decades). Mid-cap stocks tend to sit between these profiles. Many investors deliberately diversify across sizes.
Market cap vs enterprise value
Market cap measures equity value only — what equity shareholders own. Enterprise value (EV) adds net debt (total debt minus cash) to market cap, giving a cleaner picture of the total cost to acquire a business. A company with a £10bn market cap but £5bn of net debt has an enterprise value of £15bn. This matters for valuation multiples: EV/EBITDA compares the full business value to operating earnings, whereas P/E only looks at equity. When comparing companies with very different capital structures, EV-based multiples are more meaningful.
“Market cap is the market’s vote on what a company is worth today. It is not always right — but it is always the price.”
What this means for you
Market cap determines index weights: in a cap-weighted index like the S&P 500, larger companies make up a larger share of the fund. This means buying an S&P 500 fund today puts a substantial weight in a small number of mega-cap tech companies. If you want more exposure to smaller companies, mid-cap or small-cap index funds provide it. Market cap is also a quick first-pass sanity check when evaluating stocks: a startup trading at a higher market cap than an established profitable business with decades of history deserves scepticism.