If you have ever read a stock analysis, you have seen the P/E ratio. It is the simplest way to ask whether a stock is cheap or expensive relative to its profits. It is also easy to misuse.

The formula

The price-to-earnings ratio is the share price divided by earnings per share (EPS):

P/E = share price รท earnings per share

If a stock trades at $100 and the company earned $5 per share over the past year, its P/E is 20. Put another way, investors are paying $20 for every $1 of annual earnings. You can get the same result by dividing total market capitalization by total net income.

Trailing vs. forward P/E

Trailing P/E

Trailing P/E uses the last twelve months of reported earnings. It is based on real numbers but looks backward. If earnings are about to rise or fall sharply, the trailing P/E can give the wrong impression.

Forward P/E

Forward P/E uses analysts’ estimates of earnings over the next twelve months or the next fiscal year. It looks ahead, which is what stock prices do, but it is only as good as the estimates. Forecasts are frequently revised, and they tend to be too optimistic before downturns.

A balance scale weighing a price tag against a small stack of coins
The P/E ratio asks how much you pay for each dollar of profit.

What a high or low P/E means

A high P/E usually means investors expect earnings to grow quickly, so they are willing to pay more today. A low P/E can mean a stock is undervalued, but it can also mean investors expect earnings to shrink or see elevated risk. A low P/E alone is not a bargain signal.

It helps to flip the ratio. Earnings divided by price, the earnings yield, shows the profit you are buying per dollar invested. A P/E of 20 equals an earnings yield of 5%. Investors often compare this with bond yields; when Treasury yields rise, stocks with low earnings yields face more competition.

How to compare P/E ratios

  • Against the company’s own history: is it trading above or below its typical range, and has anything changed to justify that?
  • Against peers: compare companies in the same industry. Software, utilities, and banks naturally trade at very different multiples.
  • Against growth: the PEG ratio divides P/E by the expected earnings growth rate, a rough way to adjust for growth.
  • Against the market: the S&P 500’s own P/E gives a benchmark for the market as a whole.

When the P/E misleads

  • Negative or tiny earnings: a company that loses money has no meaningful P/E, and one with near-zero earnings can show an absurdly high number.
  • Cyclical companies: for chipmakers, automakers, and commodity producers, P/E often looks lowest at the peak of the cycle, when earnings are temporarily high, and highest at the bottom. See the semiconductor cycle for an example.
  • One-time items: a large gain or charge can distort a single year’s earnings.
  • Adjusted earnings: check whether a quoted P/E uses GAAP or adjusted EPS. They can differ substantially.
  • Balance sheet differences: two companies with the same P/E can have very different levels of debt and cash.

The bottom line

The P/E ratio is a starting point for a question, not an answer. Use it alongside growth, profitability, and balance sheet strength. For companies with little or no profit, see how growth stocks are valued, and for asset-heavy businesses like banks, see the price-to-book ratio.