I just noticed something that many people in the market don’t understand very well: most investors talk about the P/E ratio without really knowing what it means or how to use it. So I’m going to break this down clearly.

The P/E ratio is basically a metric that tells you how many times the company’s annual profit is reflected in its stock price. The abbreviation comes from Price/Earnings Ratio, or put another way, the relationship between the stock price and what the company earns. If a company has a P/E of 15, it means its current earnings (projected over 12 months) would pay for the entire value of the company in 15 years.

Now, calculating the P/E ratio is surprisingly simple. You have two options: divide the market capitalization by net profit, or take the stock price and divide it by earnings per share (EPS). Either way gives you the same result. The data is available on any reputable financial platform.

What’s interesting is that the P/E ratio doesn’t always work the same way. Look at Meta a few years ago: while the price was rising, the P/E ratio was systematically falling. That was a sign that they were generating more and more profits. But starting in late 2022, something different happened: the P/E ratio was falling, but the stock price was dropping too. Why? Because market reality changed. Interest rates went up, and that hit tech stocks regardless of what the P/E ratio said.

Here’s the key: you can’t rely on the P/E ratio alone. You need context. A low P/E ratio can mean the company is cheap, but it can also be a sign that its earnings are going to fall. A high P/E ratio can indicate that the market sees enormous potential, or that we’re in a speculative bubble.

Analysts often prefer a P/E ratio between 10 and 17. It’s considered a comfort zone for expecting growth without negative surprises. Below 10, it attracts value investors, but be careful: sometimes the P/E ratio is low because the company is a complete disaster. Above 25, we’re in “wagering” territory: either the market sees a bright future, or all of us are crazy.

One crucial point: the P/E ratio varies a lot depending on the sector. Technology or biotech companies can have P/Es of 100 or more without it being strange. But a bank or an industrial company with a P/E of 100 would be madness. You have to compare P/Es with P/Es.

Something people forget is that the traditional P/E ratio only looks at one year of earnings, which can distort reality. That’s why the Shiller P/E exists: it takes the average earnings of the last 10 years adjusted for inflation. Theoretically, that gives you a more accurate picture of what’s really going on.

If you’re going to use the P/E ratio to invest, do it properly. Combine it with other ratios: ROE, ROA, and the price relative to book value. Look at the actual composition of the business. Sometimes earnings come from selling an asset, not from the company simply making more money. That changes everything.

In summary, the P/E ratio is a practical, easy-to-calculate tool—perfect for quick comparisons within the same sector. But if you base your entire investment strategy only on the P/E ratio, you’ll fail. There are too many variables. What works is using the P/E ratio as a starting point, then digging into the company’s real numbers, understanding which sector it operates in, and what economic cycle it’s going through. Only then can you build something solid.