P/E ratio explained: how to value a stock

The P/E ratio (Price-to-Earnings) is the most widely used valuation metric in stock investing. It tells you how much investors are paying for every dollar (or euro) of a company's earnings. Used correctly, it's a powerful screening tool — but it's also the most frequently misinterpreted ratio, which makes understanding its nuances essential.

  • P/E = Stock Price / EPS — how much you pay per €1 of earnings
  • S&P 500 long-run average: 15–17×. Below 15 may be cheap; above 30 needs justification
  • Compare P/E within the same sector — tech P/Es are structurally higher than utilities
  • Low P/E can be a value trap if earnings are at a cyclical peak or trending down
  • PEG ratio (P/E ÷ growth rate) adjusts for growth: PEG below 1 can indicate value
  • Rising interest rates compress fair P/E multiples across the whole market

How to calculate the P/E ratio

P/E = Stock Price / Earnings Per Share (EPS). If a stock trades at €60 and the company earned €4 per share over the past 12 months, P/E = 60 / 4 = 15. This means investors are paying €15 for every €1 of earnings — or equivalently, paying 15 years of current profits for ownership of the business. Trailing P/E uses the last 12 months of actual reported earnings. Forward P/E uses analyst consensus estimates for the next 12 months. Forward P/E is more relevant for growing companies where future earnings matter more than the recent past.

How to interpret P/E ranges

Historically, the S&P 500 has traded at a long-run average P/E of around 15–17. A P/E below 15 can suggest undervaluation or low growth expectations. A P/E above 25–30 suggests high growth expectations are already priced in. Tech companies routinely trade at 30–50× earnings because of high growth rates; utilities at 12–16× because growth is stable and modest. These aren't firm rules — they're starting points. Context always matters more than the absolute number.

P/E by sector: compare apples to apples

Comparing the P/E of a tech company to the P/E of a bank is meaningless. Each sector has its own "normal" range based on typical growth rates and business models. Technology: 25–50×. Consumer staples: 20–30×. Healthcare: 20–35×. Financials: 8–15×. Utilities: 12–18×. Energy: 10–15×. Always compare a company's P/E to its direct peers and to its own historical range. A P/E of 25 might be cheap for a cloud software company growing at 30%/year and expensive for a mature retailer growing at 3%.

When a low P/E is a value trap

A very low P/E can signal value — or a trap. The most common traps: cyclical companies at peak earnings (the P/E looks low because current profits are temporarily inflated — think commodity producers at the top of a cycle); businesses losing competitive position (earnings today are still high but trend is clearly down); and companies where accounting earnings are high but cash generation is poor. Always check earnings sustainability: is the trend up, flat, or declining? Is the business model still intact?

The PEG ratio: adjusting P/E for growth

The PEG ratio = P/E / Annual Earnings Growth Rate. It adjusts the P/E for the company's growth speed. A P/E of 30 for a company growing earnings at 30%/year gives a PEG of 1.0. A P/E of 30 for a company growing at 10%/year gives a PEG of 3.0 — much less compelling. As a rough rule: PEG below 1.0 can indicate the stock is undervalued relative to its growth. PEG above 2.0 suggests you're paying a large premium for that growth. Peter Lynch, who popularized the PEG ratio, considered a PEG of 0.5 or below a strong buy signal.

Interest rates and P/E: the macro context

Interest rates directly affect what P/E is "fair" for the market. When rates are low (0–2%), investors accept higher P/Es because bonds offer little competition. When rates rise to 4–5%, a risk-free bond at 5% competes with a stock at P/E 20 (implying a 5% earnings yield). The market P/E naturally compresses when rates rise. This is why the same P/E of 20 that was reasonable in 2020 (rates near zero) was considered stretched in 2023 (rates at 5%). Always interpret individual P/Es in the context of the prevailing interest rate environment.

Frequently asked questions

What is a good P/E ratio?

There is no universal number: it depends on the sector, expected growth and interest rates. Compare with peers and the company's own history.

Does a low P/E mean a stock is cheap?

Not necessarily. It may reflect business problems or earnings about to fall.

What is forward P/E?

Price divided by expected earnings for the next 12 months. Useful, but depends on forecasts.

What is the PEG ratio?

P/E divided by the earnings growth rate. It helps compare companies growing at different speeds.

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