Market Capitalisation Explained, and What It Leaves Out

Market capitalisation is share price multiplied by shares outstanding. It is quoted constantly, used to rank companies and to sort them into indexes, and it is genuinely useful. It is also narrower than most readers assume.

The number describes one thing precisely: what the market says the equity is worth. It does not describe what the business is worth, and the difference between those two statements is where most misreadings live.

Equity is a residual claim

A company is funded by some mix of equity and debt. Debt holders have first claim on cash flows and on assets in a liquidation. Equity holders get whatever is left.

So market cap prices the residual. Two companies running identical operations, generating identical cash, can have very different market capitalisations if one is financed largely by debt and the other by equity. The businesses are the same; the slice that shareholders own is not.

Enterprise value asks the fuller question

Enterprise value is the standard correction. It takes market cap, adds debt, and subtracts cash, approximating what it would cost to acquire the whole business — you would buy the equity, assume the obligations, and get the cash on the balance sheet.

The cash adjustment surprises people. A company with a large cash pile has an enterprise value well below its market cap, because a buyer would immediately have that cash. On that measure the operating business is cheaper than the headline suggests.

This is why acquisition prices are usually discussed in enterprise value terms while stock coverage uses market cap. They answer different questions: what would it cost to buy the company, versus what are the shares worth.

Which shares are counted

Shares outstanding sounds unambiguous and is not. Employee options and restricted stock will become shares. Convertible bonds may. A fully diluted count includes these; a basic count does not, and the gap can be material at companies that pay heavily in equity.

Index providers often use free float — only shares actually available to trade, excluding stakes held by founders, governments or cross-holdings. A company can therefore have a large market cap and a much smaller index weight.

The comparisons that go wrong

Market cap invites comparisons it does not support.

Why it still gets used

For all that, market cap earns its place. It is observable continuously, requires no assumptions, and cannot be adjusted by an accounting choice. Enterprise value requires balance sheet data that arrives quarterly and is subject to judgment about what counts as debt. Market cap is available now and means the same thing for everyone.

It is also the basis on which most index funds allocate, which makes it self-reinforcing: a company's index weight, and therefore a portion of demand for its shares, follows from it.

How market cap shapes index membership

Because most index funds weight holdings by market capitalisation, a company's size determines how much passive money must hold it. That relationship has consequences worth understanding.

When a company is added to a major index, funds tracking that index must buy it, regardless of price. When it is removed, they must sell. These are mechanical flows driven by rules rather than by any view on value, and they can move prices meaningfully around the date of a change.

Over time it produces a subtler effect: as a company grows, its index weight grows, which increases passive demand for its shares, which supports its price. Critics argue this makes cap-weighted indexing momentum-following by construction — it buys more of what has already risen. Defenders point out that it requires no trading to maintain, which keeps costs low, and that any alternative weighting embeds its own judgment.

Multiples, and reading them properly

Market cap is the numerator in most valuation multiples, which is where it does most of its analytical work.

A multiple is not a verdict. A high one can mean a company is expensive or that it is expected to grow; a low one can mean it is cheap or that its earnings are about to fall. The multiple tells you what the market is currently assuming, and the analytical work is deciding whether that assumption is reasonable.

This is also why comparing multiples across industries misleads. A business requiring heavy ongoing capital investment genuinely deserves a lower multiple than one that does not, because less of its reported profit ends up as cash available to owners.

How to use it

Treat market cap as a starting point rather than a conclusion. It tells you the scale of the equity claim and roughly where a company sits in the market's hierarchy. For anything comparative — is this expensive, how does it compare to a peer, what would an acquirer pay — reach for enterprise value and the underlying cash flows.

The number is honest about what it measures. The trouble starts when readers ask it to measure something else.