How to interpret the PER in stocks: a practical guide to avoid falling into the trap

The P/E ratio is probably the most popular indicator among investors, but also one of the most misunderstood. Many believe that just looking at this number is enough to find a “bargain” in the stock market. The reality is much more complex, and understanding its limitations is as important as knowing how to calculate it.

The P/E ratio explained without jargon: what does it really measure

The acronym P/E stands for Price/Earnings Ratio. In simple terms: it shows how many years of current profits a company would need to match its market value.

If a company trades at a P/E 15, it means that by buying shares of that company, you would pay for 15 years of its profits. In other words, the P/E relates the price of each share to its earnings per share (EPS).

Although it seems simple, the P/E does not always operate the same way. In some cases, you’ll see that when the P/E drops, the stock rises (indicating it earns more money while maintaining the price). In others, the stock falls even as the P/E decreases because external factors (such as changes in interest rates) dominate the market. The example of Meta Platforms illustrates this well: after the end of 2022, its shares fell despite an improved P/E because investors shied away from tech stocks.

Calculating the P/E in stocks: two equivalent formulas

The calculation is straightforward. You can use either of these two options:

Method 1 (overall figures): Market capitalization ÷ Net profit = P/E

Method 2 (per share): Share price ÷ Earnings per share (EPS) = P/E

Both give the same result. The data is accessible on any financial platform like Yahoo Finance or Infobolsa, where the P/E appears alongside EPS, market cap, and other indicators.

Practical example: a company with a market cap of $2.6 billion and profits of $658 million has a P/E of 3.95. In another case, if a share costs $2.78 and its EPS is $0.09, the P/E is 30.9. Notice the huge difference: the first seems cheap, the second expensive.

What each P/E range really means

Interpretation depends on the context, but here’s what it usually signifies:

• P/E between 0-10: Apparently attractive, but beware: it may indicate that profits will fall soon. Companies in crisis often have low P/E because no one trusts them.

• P/E between 10-17: The “fair” zone according to analysts, where moderate growth is expected without extreme risks.

• P/E between 17-25: Sign of recent growth or possible overvaluation. Requires further investigation.

• P/E above 25: Here there are two stories: either the company has very positive prospects, or we are in a speculative bubble. Tech and biotech companies tend to be here, while banks and heavy industry typically have low P/E.

The traps of the P/E: why you shouldn’t use it alone

One critical weakness is that the P/E of stocks only looks at profits from one year. If that was an exceptional (extraordinary year), the indicator can deceive you. Arcelor Mittal (steel) has a P/E of 2.58, while Zoom Video reached 202.49. Should I buy Arcelor? Not necessarily. Comparing P/E across different sectors is a common mistake. The correct approach is to compare companies within the same sector under similar market conditions.

Another trap: the P/E is inapplicable to companies that do not generate profits. Loss-making startups can have an undefined P/E. Additionally, it does not reflect changes in profit quality. A profit can grow because the company sells an asset, not because of better operations.

Alternatives and complements to the P/E in stocks

For more robust analysis, combine the P/E with:

• Shiller P/E: Uses 10-year average profits adjusted for inflation, reducing distortions from atypical years
• Normalized P/E: Adjusts for debt and liquid assets, providing a more faithful picture of the company’s financial health
• Other ratios: EPS, ROE, ROA, Price/Book Value

Value Investing is built on the P/E, but not exclusively. Funds like Horos Value Internacional have a P/E of 7.24 (low), but that is part of a comprehensive strategy, not their only criterion.

What makes the P/E useful, and what limits it

Advantages:

• Simple calculation and accessible data
• Excellent for quick comparison between similar companies
• Works even when there are no dividends
• Benchmark indicator for professional investors

Limitations:

• Only captures one year’s profits
• Useless for unprofitable companies
• Static, not dynamic
• Especially misleading in cyclical companies (low at cycle peaks, high at cycle troughs)

The important conclusion

The P/E is a valuable tool, but it is just that: a tool. Do not make decisions based solely on it. Many companies with low P/E failed because of disastrous management. Profitable investing requires combining the P/E with sector analysis, cost structure, competitive advantages, and market trends. Spend at least 10 minutes understanding the true quality of the company before investing.