Not all stocks are the same. A share in Apple and a share in a small mining company both trade on exchanges, but the similarities end there. The size of a company — measured by market cap — is one of the most useful lenses for understanding what you're buying, how risky it is, and how it tends to behave in different market conditions.
Market cap = share price × total shares outstanding. It tells you what the entire company is worth at today's market price. Companies are grouped into size categories that matter enormously for how they trade, how much information is available on them, and what kind of returns and risks they carry.
| Category | Market Cap | Examples |
|---|---|---|
| Mega cap | $200bn+ | Apple, Microsoft, Nvidia, Saudi Aramco |
| Large cap | $10–200bn | Rolls-Royce, Next, Legal & General |
| Mid cap | $2–10bn | Most FTSE 250 companies |
| Small cap | $300m–2bn | Smaller listed companies |
| Micro cap | $50–300m | Very small listed companies |
| Penny stocks | Under $50m, often <£1/share | Highly speculative, illiquid |
Large-cap stocks are heavily researched by analysts, widely held by institutional investors, and highly liquid — you can buy and sell large amounts without moving the price significantly. They tend to be more stable in downturns. The trade-off: because they're so thoroughly analysed, genuine mispricing is rare. The edge is harder to find.
Smaller companies can grow faster — a company going from £50m to £200m market cap is a 4× return. The same growth in percentage terms is far harder to achieve from a £100bn base. But small caps are less liquid, less researched, more volatile, and more vulnerable in economic downturns. Many have limited track records, concentrated management teams, and can be significantly affected by a single contract win or loss.
The small-cap research gap: FTSE 100 companies have dozens of analysts covering them. Many small-cap companies have one or two, or none at all. This creates genuine mispricing opportunities for investors willing to do their own research — but it also means you have far less information to work with, and the companies themselves are harder to scrutinise.
Blue chips are large, established, financially stable companies with long track records of reliable earnings — typically household names that have survived multiple recessions. In the UK: Unilever, AstraZeneca, HSBC, BP. In the US: Johnson & Johnson, Coca-Cola, Procter & Gamble. They're not immune to falling, but they're the companies most likely to still be operating in 30 years.
Penny stocks trade at very low prices (often under £1 or $1) and usually have tiny market caps. They're highly illiquid, difficult to research, loosely regulated in many cases, and frequently the target of pump-and-dump schemes. The very low price makes them feel cheap — but price per share alone tells you nothing about value. A stock trading at 5p is not "cheap" just because 5p sounds small.
For most investors: start with large and mid caps where information is plentiful and liquidity is good. As your knowledge grows, small caps become interesting — but only once you understand what you're actually researching and why the market might have undervalued something others have missed.
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