Market Capitalisation | Investing Glossary

What it is

Market capitalisation — almost always shortened to market cap — is simply the current share price multiplied by the number of shares in issue. If Vodafone has 27 billion shares and they trade at 68p, the market cap is approximately £18.4 billion.

That's what the market, right now, thinks the equity of the business is worth. Nothing more.

How to calculate it

**Market Cap = Share Price × Shares Outstanding**

The share count should be the fully diluted figure — including potential shares from options, warrants, and convertible securities — for the most conservative view.

Market cap vs enterprise value

This is where many investors go wrong. Market cap is the value of the equity. It ignores the debt the company owes. Two companies can have identical market caps but vastly different total values — if one has £500m of net debt and the other has £500m of net cash, the actual cost of buying each business differs by £1 billion.

Enterprise value — market cap plus net debt minus cash — is the more complete measure of what a business is worth, and why M&A deals are priced on EV rather than market cap.

Market cap categories

Category	UK definition	Examples
Large cap	£5bn+	Shell, AstraZeneca, HSBC
Mid cap	£1bn–£5bn	Watches of Switzerland, Future Plc
Small cap	£150m–£1bn	Many FTSE Small Cap constituents
Micro cap	Under £150m	AIM-listed growth companies

The FTSE 100 is an index of the 100 largest companies by market cap listed in the UK. The FTSE 250 covers the next 250. AIM (Alternative Investment Market) is home mainly to smaller and earlier-stage companies.

Why size matters for investment

Larger companies tend to be more liquid — easier to buy and sell without moving the price. They're also more heavily researched by institutional analysts, which means pricing inefficiencies are rarer.

Small and micro-cap stocks can offer greater upside precisely because they're under-researched and under-owned. A fund manager running £10 billion cannot meaningfully invest in a £50m company — the position would either be too large to enter without moving the price, or too small to affect the fund's returns. This creates inefficiency that private investors can exploit.

The flip side is liquidity risk. In a falling market, small-cap stocks can fall much harder than large caps because there are fewer buyers prepared to step in. The bid-offer spread (the gap between buying and selling prices) also widens dramatically for illiquid stocks in volatile conditions.

Market cap is not the same as value

This deserves repeating. A company with a £20 billion market cap is not necessarily worth more than a company with a £5 billion market cap. It depends on earnings, growth, cash flows, and the price being paid for those.

A bloated large-cap trading at 40x earnings may be considerably more expensive — in the sense of what you're paying per pound of earnings — than a mid-cap trading at 10x earnings. Market cap is a price. Value is a different calculation entirely.

**Useful context:** As of 2025, AstraZeneca is the UK's largest listed company by market cap at approximately £175bn. The smallest FTSE 100 company typically has a market cap of around £4–5bn. The average market cap of a UK-listed small cap is £300–500m.

Not financial advice. Market cap data changes daily as share prices move.