TL;DR Market cap is the price of one share multiplied by the number of shares that exist — the sticker price of the entire company.
The plain-English version
A share price alone tells you almost nothing, because companies slice themselves into wildly different numbers of shares.
Company A: $900 per share, 1 million shares → the whole company is priced at $900 million. Company B: $50 per share, 1 billion shares → the whole company is priced at $50 billion.
Company B's stock looks "cheaper" but the business is priced at more than 50 times Company A. That total — share price × share count — is the market capitalization.
The size classes
Investors sort companies into weight divisions:
- Mega cap — hundreds of billions to trillions. The giants everyone can name.
- Large cap — roughly $10B and up. Established, widely covered.
- Mid cap — around $2B–$10B. Grown up, still growing.
- Small cap — around $250M–$2B. More room to run, more ways to die.
- Micro/nano cap — below that. The wild west; where most scams live.
The common mistake
Thinking a low share price means room to grow. "It's only $2, if it just goes to $10 that's 5x!" — but if that company has two billion shares, it's already priced at $4 billion, and going to $10 means the market valuing it at $20 billion. Ask whether the company can plausibly be worth the new market cap, not whether the number looks small.
This is also why penny stocks aren't lottery tickets with better odds. The price per share is low because of the share count, not because the company is undiscovered.
Why you care
Market cap tells you what the market believes an entire business is worth, what index funds will include it, how volatile it's likely to be, and whether a headline number is impressive or laughable. It's the first number to check after the ticker.
Educational only — not investment advice. Big isn't safe and small isn't destiny; market cap is context, not a verdict.