P/E Ratio (Price-to-Earnings Ratio)
Definition
The P/E ratio is a valuation measure comparing a company's share price to its earnings per share, showing how much investors are paying for each dollar of profit.
Detailed Explanation
The calculation is share price divided by earnings per share (EPS). A stock trading at $60 with EPS of $3 has a P/E of 20 (investors are paying $20 for every $1 of annual profit). One useful way to read that number is as a rough payback period: at current earnings, it would take 20 years of profit to equal the price. That framing makes clear why P/E is a valuation measure rather than a quality measure. It tells you what you're paying, not what you're getting.
P/E Ratio = Share Price / Earnings Per Share
Which earnings you use changes the answer. Trailing P/E uses the last twelve months of actual reported earnings (factual but backward-looking). Forward P/E uses analysts' estimates for the next twelve months, which is more relevant to what you're buying but depends on forecasts that are often wrong.
Financial sites default to different ones, so two sources can show different P/Es for the same stock on the same day. Check which is which before comparing. A related variant, the CAPE or Shiller P/E, averages ten years of inflation-adjusted earnings to smooth out the business cycle, and is more often applied to whole markets than to individual companies.
Context is everything, and this is where the ratio gets misused. A P/E of 12 isn't cheap and 40 isn't expensive without knowing the industry and the growth rate. Utilities and banks typically trade at low multiples because their earnings grow slowly while software companies trade high because investors are pricing in growth that hasn't arrived yet.
Comparing a company to its own history and to direct competitors is informative. Comparing across sectors usually isn't. This is why P/E anchors value investing screens but never functions as the sole criterion.
The ratio also breaks in specific, common situations. A company with negative earnings has no meaningful P/E — the figure is either blank or nonsensical, which affects most early-stage and many recently public companies. A one-time gain or charge can distort the denominator badly for a year, making a stock look artificially cheap or expensive. And earnings themselves are an accounting output shaped by real choices about depreciation, reserves, and revenue timing, all visible in the financial statements. A low P/E sometimes signals a bargain and sometimes signals that the market expects earnings to fall (the ratio can't distinguish between the two) which is the origin of the phrase "value trap."
Example
Suppose two companies both trade at $80 per share. Company A earned $4.00 per share last year, giving it a P/E of 20. Company B earned $1.60, giving it a P/E of 50. Company B looks far more expensive per dollar of current profit, but if B's earnings are growing 40% a year and A's are flat, the market's pricing may be entirely rational. Now suppose Company A's $4.00 included a one-time $1.50 gain from selling a building. Its real operating P/E is closer to 32, not 20. Same headline number, very different conclusion.
Key Articles Related To P/E Ratio
Related Terms
Earnings Per Share (EPS): A company's profit divided by its outstanding shares, and the denominator of the P/E ratio.
Earnings Report: The quarterly financial results a public company releases, which update the earnings figure P/E depends on.
Forward P/E: The ratio calculated using projected earnings for the coming year rather than reported past earnings.
PEG Ratio: The P/E ratio divided by the earnings growth rate, an attempt to adjust valuation for how fast a company is growing.
Book Value: The accounting value of a company's assets minus liabilities, used in the price-to-book ratio as an alternative valuation measure.
FAQs
How do you calculate the P/E ratio?
Divide the current share price by earnings per share. A $60 stock with $3.00 in EPS has a P/E of 20. Most financial sites calculate it for you, but check whether they're using trailing or forward earnings.
What is a good P/E ratio?
There's no universal threshold. What counts as reasonable depends on the industry, the company's growth rate, and interest rates. The useful comparisons are against the company's own history and against direct competitors — not against the market as a whole or against companies in unrelated sectors.
What's the difference between trailing and forward P/E?
Trailing P/E uses the last twelve months of reported earnings, which are facts. Forward P/E uses analyst estimates for the next twelve months, which are predictions. Forward P/E is generally lower for growing companies, since the projected earnings are larger.
Does a low P/E mean a stock is undervalued?
Not necessarily. A low multiple can mean the market has overlooked a solid business, or it can mean the market expects earnings to decline. The ratio alone can't tell you which. Buying a cheap-looking stock whose earnings then deteriorate is common enough to have a name — a value trap.
What if a company has no P/E ratio?
It almost always means the company isn't profitable, so the denominator is zero or negative and the ratio has no meaning. Many young and recently public companies fall into this category, which is why investors use revenue-based measures like price-to-sales for them instead.
Can you use P/E for an index or the whole market?
Yes, and index-level P/E is widely used to judge whether the broad market looks expensive relative to history. Because single-year earnings swing with the business cycle, the CAPE ratio — which averages a decade of inflation-adjusted earnings — is often preferred for that purpose.