P/E is price ÷ earnings — and both halves move.
- 22× means paying 22 years of today’s profit
- Falling profit RAISES the P/E — “cheap” can be a trap
- P/E cannot see debt
What this is about
The price-to-earnings ratio is the most quoted number in investing and the most casually misused. It is genuinely useful — and almost every way people use it in conversation is one of the traps below.
Worked example continues with Northwind Components, fictional.
What it is
Share price £34.00 Earnings per share £1.525 (61m profit ÷ 40m shares) P/E 22.3×
Two readings, both correct:
"You are paying 22.3 times one year's earnings."
"At this profit level, it would take 22.3 years of earnings to repay the price." — the more sobering phrasing, and the more useful one.
Flip it and you get the earnings yield: 1 ÷ 22.3 = 4.5%. That number is directly comparable to a bond yield or a savings rate, which is what makes it worth calculating.
Trap 1: comparing across industries
Northwind Components (industrial) 22.3× Cascade Grocers (supermarket) 14.1× Aurora Systems (software) 46.8×
Aurora is not "expensive" and Cascade is not "cheap". Different industries carry different ratios for structural reasons: growth rates, how much capital they must reinvest, how predictable earnings are, how much debt is normal.
A P/E is only informative against a comparable. Against the same company's own history, and against direct competitors. Ranking unrelated businesses by P/E produces a list, not an insight.
Trap 2: the denominator is one year
E is a single year's earnings, and a year can be unrepresentative.
A bad year inflates it. Suppose Northwind has a poor 2026 — profit falls to 20m on a one-off write-down. Price unchanged:
P/E = £1,360m ÷ £20m = 68×
The company now looks wildly expensive on a number that says nothing about its normal earning power.
A peak year deflates it. A cyclical business at the top of its cycle shows a low P/E precisely when its earnings are least sustainable — which is the single most expensive P/E mistake there is, because the number looks most attractive exactly when it is least reliable.
This is the origin of the phrase value trap: a low P/E that reflects earnings about to fall, not a bargain.
Trap 3: which E
You will see several, and they are not interchangeable:
- Trailing — the last twelve months. Factual, and backward-looking.
- Forward — analysts' estimate for next year. Useful, and an estimate, and estimates are systematically optimistic on average.
- Adjusted — the company's own preferred figure, excluding items it considers one-off. Sometimes reasonable, sometimes a way of excluding costs that recur every year under different names.
A stated P/E without saying which E is an incomplete number. Trailing and forward on the same company routinely differ by a third.
Trap 4: it ignores the balance sheet
Two companies, identical earnings and identical P/E. One has no debt; the other has debt equal to half its market value. You are not buying the same thing, and P/E cannot see the difference. Debt belongs in the judgement separately — it is step 5 of lesson 1 for exactly this reason.
Where it is genuinely useful
Against the company's own history. Northwind at 22× when it has averaged 15× over five years is a real observation: you are paying more than buyers usually have. Whether that is justified is the question, and now it is the right question.
As an earnings yield, against alternatives. 4.5% against what a government bond pays is a comparison with actual content.
As a statement of what must happen. A high P/E is the market pricing in growth. Writing down what growth it implies turns a vague "expensive" into something checkable.
What this does not tell you
It does not tell you whether a company is good. Excellent businesses trade at high multiples and bad ones at low. P/E measures price against one year of profit, and nothing about quality.
It does not tell you a low P/E is cheap. Often it is the market's judgement that earnings will fall. Sometimes the market is wrong; assuming so without a reason is not analysis.
It does not work on companies with no earnings. Negative E makes the ratio meaningless, not infinite — which is why loss-making growth companies get judged on other measures.
It does not stay comparable through a share issue or buyback. Both move EPS without the business changing.
Where to see this in the app
Chart → Key stats carries the P/E alongside market cap, so step 6 of lesson 1 is one screen.
Chart → Financials is where you check the denominator before trusting the ratio — a glance at four years of net income tells you whether this year's E is representative or an outlier, which is trap 2 defused in about fifteen seconds.
Watchlists is the honest way to use P/E: put three companies from the same industry side by side. That is the only comparison the number supports.
Chart → Analyst rating shows published expectations, which is where a forward P/E's E comes from — worth knowing before relying on one.