What is a stock's P/E ratio?
Updated on August 24, 2026 · by Alex
The P/E ratio (price-to-earnings) divides a stock's price by the earnings that stock generates in a year. If a stock trades at 100 and earns 5 a year, its P/E is 20: you are paying twenty years of current earnings for it.
The P/E answers a simple question: how many years of current earnings you are paying. A P/E of 20 means that, if the company kept earning exactly the same every year, it would take you twenty years to get back what you paid.
Flipped around, it is a thermometer for expectations. A high P/E doesn't say the stock is expensive: it says the market expects earnings to grow. Nobody pays 40 years of earnings for a company that will earn the same forever; they pay it because they believe it will earn double in three years.
It is the fastest way to compare two companies in the same sector. If two similar banks trade at a P/E of 9 and a P/E of 15, the interesting question is what the market sees in the second one that it doesn't see in the first — or what risk it sees in the first that it doesn't see in the second.
The key words are in the same sector. Comparing the P/E of a tech company with the P/E of a utility tells you nothing: they are businesses with completely different growth rates and capital needs. A P/E of 12 is high for a miner and rock-bottom for a software company.
Assuming a low P/E means cheap. It is the most common mistake and the most expensive one.
A low P/E usually means one of three things: that the market expects earnings to fall (so the real future P/E is much higher than it looks), that the business is in a declining industry, or that there is a problem the numbers haven't shown yet. This is what gets called a value trap: the stock looks cheap for years, and keeps looking cheap while the business deteriorates.
The other mistake is using the P/E on companies that have no earnings. If earnings are zero or negative, the P/E either doesn't exist or spits out a meaningless number. In those cases you have to look at other things: revenue growth, gross margin, cash flow.
Rule of thumb: the P/E is not a conclusion, it is a question. An odd number — very high or very low — tells you where to look, not what to do.
