How a Share Gets Priced: Supply, Demand, Market Cap, P/E

ℹ️ This is education, not investment advice. It teaches you how a price is built and what it encodes — not which stock to buy. A low ratio is not a "buy" signal, and this lesson never gives one.

A $6 stock is not cheaper than a $600 stock

Open two stocks. One trades at $6, the other at $600. Almost everyone's gut says the first is "cheaper." It's the instinct that makes people buy a hundred shares of the $6 one and feel like they got more.

They got more pieces. They did not get more company. The price of a single share, on its own, tells you almost nothing — because a company can slice itself into any number of shares it likes. The same business can be $6 × 10 billion shares or $600 × 100 million shares. Same company, same value, wildly different sticker.

In MN-01 you saw where the price comes from — the order book, moment to moment. This lesson is about what the price means: how big the company actually is, and what the market is quietly predicting when it sets that number.

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Why nobody explained it this way

Because the number everyone shows you is the wrong one. Apps quote the share price. News tickers scroll the share price. "It's up 3%" is the share price. It's the most visible number and close to the least informative in isolation.

The two numbers that actually describe a company — how big it is, and what's expected of it — take one extra step to compute, so they get skipped. That step is this lesson.

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1. Market cap: the only "size" that means anything

To compare two companies you have to compare the whole of each, not one slice.

Market capitalisation = share price × total shares outstanding

"Shares outstanding" is how many slices exist. It's a real, reported number — you'll find it on the company's own filing (its 10-K in the US) and on any finance site's summary page.

The "cheap" $6 stock is a company twice the size of the $600 one. Price per share told you the opposite of the truth. This is why market cap — not price — is how companies get sorted into large-cap, mid-cap and small-cap, and how most big indices decide how much of each to hold.

Buying-the-whole-company test. Any time a share price tempts you, multiply it by shares outstanding and ask: would I buy this entire business for that? It reframes the number instantly.

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2. Supply and demand set the price; earnings anchor it

Moment to moment, the price is pure supply and demand — the order book from MN-01. More buyers than sellers at the current price, the price ticks up until enough sellers appear. That's the short-run engine, and it's why prices twitch all day on no real news.

But over months and years, a share can't drift completely free of the business underneath it. What tethers it is earnings — the actual profit the company makes. A price is, loosely, the market's bet on future earnings. So the useful question isn't "is the price high?" It's "high relative to what the company earns?" That ratio has a name.

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3. The P/E ratio: price measured in years of profit

The price-to-earnings ratio puts price next to profit so two companies of different sizes become comparable.

P/E = market cap ÷ annual net income (identically: share price ÷ earnings per share)

Both forms give the same number. Net income is the company's annual profit after everything — costs, interest, tax — and sits at the bottom of the income statement (hence "the bottom line").

Read the result as years: a P/E of 20 means that, at today's price and today's profit, buying the whole company would take 20 years of its current earnings to pay you back.

Now the picture flips again. The bigger company (A) earns less and costs more per dollar of profit. The market is paying 30 years of A's earnings and only 10 of B's. Why would anyone do that?

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4. What a P/E actually encodes: expectation

A P/E is not a measure of quality. It's a measure of what the market expects next, and it's driven by three things:

• Growth. A high P/E means the market expects earnings to grow — so today's profit understates the future, and paying 30× today looks reasonable if profit is about to multiply. A low P/E often means the market expects earnings to stall or shrink.
• Risk. The less certain those future earnings, the less the market will pay for them today. Same profit, riskier business, lower P/E.
• Interest rates. From MN-01: when rates rise, future money is worth less today, so the price every dollar of future earnings commands falls. Rates up, P/Es down, across the whole market — with nothing changing at any single company.

So the number is a compressed forecast. Company A at 30× isn't "overpriced" and Company B at 10× isn't "a bargain." The market is saying: I expect A to grow and B to stall. The P/E is the question — "what does the market believe here, and do I think it's wrong?" — not the answer.

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5. The two traps this sets

Trap one: "low P/E = cheap." Sometimes a low P/E is a company the market has correctly judged to be in decline. The profit is real today and evaporating tomorrow. Buying it because the ratio looks low is walking into a value trap — cheap for a reason.

Trap two: "high P/E = overpriced." Sometimes a high P/E is a company whose earnings genuinely go on to multiply, and the "expensive" price turns out to have been a discount. Selling on the ratio alone misses it.

Both traps come from treating the P/E as a verdict. It isn't. It's the market's expectation made visible — useful precisely because it tells you what you'd be betting against. A stock with a P/E of 40 is a company where you only make money if it grows even faster than an already-optimistic crowd expects.

One caution before the exercise: if a company loses money, net income is negative and the P/E is meaningless (you'll see "N/A" or a negative number). No profit, no ratio — a different toolkit applies there, and that's MN-06.

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What this means for you

You can now look at a single stock and read three things off it that price alone hid: how big the company really is (market cap), how many years of profit you're paying for (P/E), and therefore what the market is predicting (growth, stagnation, or risk). That's the difference between "the number went up" and understanding what the number is.

It doesn't tell you what to buy — nothing here does. It tells you what the price is saying, so that if you ever act, you're arguing with a specific expectation instead of guessing at a chart.

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Try it: read one real stock (25 min)

Costs nothing. Turns a price into a company.

1. Pick a well-known, profitable company from a major index. Find four numbers on its summary page (or its latest 10-K): share price, shares outstanding, net income (annual), and the site's stated P/E.
2. Calculate market cap yourself: price × shares outstanding. Does it match the "market cap" the site shows? (It should, within rounding.)
3. Calculate the P/E yourself: market cap ÷ net income. Does it match the site's figure? If it's off, the site is likely using forward (expected) earnings, not trailing (last year's) — note which.
4. Interpret the number in years: "at today's profit, the whole company pays back in ___ years."
5. Now pick a direct competitor and do steps 1–4. One will have a higher P/E.
   • Write one sentence: what is the market expecting from the higher-P/E company that it isn't expecting from the other — faster growth, or lower risk?
6. Finally, find one company with a negative or missing P/E and confirm it's because it isn't profitable yet.

✅ Finish check: you have, for two competitors, a market cap and a P/E you computed by hand, plus one written sentence naming the expectation the market has priced into the more expensive one.

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Summary card

• Share price alone is meaningless — a company can cut itself into any number of shares.
• Market cap = price × shares outstanding is the only real measure of size. A $6 stock can be bigger than a $600 one.
• Short run, price is supply and demand; long run, it's tethered to earnings.
• P/E = market cap ÷ net income, read as years of profit to pay back.
• A P/E encodes expectation: high = growth expected, low = stagnation or risk, and rates move all of them at once.
• Low P/E isn't automatically cheap (value trap) and high P/E isn't automatically expensive (real growth). The ratio is the question, not the answer.
• No profit, no P/E — an unprofitable company needs a different lens (MN-06).

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Sources

1. U.S. Securities and Exchange Commission — Investor.gov: Market Capitalization and Price-Earnings Ratio
2. SEC — Form 10-K (reported shares outstanding and net income)
3. Damodaran, A. — Investment Valuation (NYU Stern); the drivers behind earnings multiples
4. S&P Dow Jones Indices — Index Mathematics Methodology (float-adjusted market-cap weighting)

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Next lesson: MN-06 — Reading a Company's Financials: Balance Sheet and Income Statement (L3) Related: MN-01 What Are You Actually Buying (L1) · MN-07 Inflation, Interest Rates and Currency (L2) · MN-05 Risk, Volatility and Diversification (L2) Path: Company Reader — 1/5