Price-to-Earnings (P/E) Ratio Calculator

The price-to-earnings ratio divides a stock's market price per share by its earnings per share (EPS). The formula is P/E = share price / EPS. A stock trading at $150 with $6 of trailing twelve-month EPS has a P/E of 25, meaning investors are paying $25 today for every $1 of recent annual profit. The same math runs in reverse as the earnings yield: 1 / P/E expressed as a percent, which lets you compare equities directly against bond yields. A P/E of 25 corresponds to a 4 percent earnings yield.

Trailing P/E (often labeled TTM, trailing twelve months) uses the last four reported quarters of GAAP earnings. It is backward-looking but objective, since the numbers have already been audited and filed in 10-Q or 10-K reports with the SEC. Forward P/E uses analyst consensus estimates for the next twelve months of EPS as the denominator. It is more relevant for fast-growing companies, where TTM understates earning power, but it is only as reliable as the estimates themselves. Sell-side consensus tends to be too optimistic at cyclical peaks and too pessimistic at troughs. A useful habit is to look at both: a stock cheap on forward P/E but expensive on trailing P/E is essentially a bet that the growth forecast will be realized.

Robert Shiller's CAPE dataset, which tracks the S&P 500 (and its predecessor indices) since 1881, shows a long-run trailing P/E median of about 16 and a mean closer to 17. The trailing P/E spends most of its time between 12 and 22, with sharp spikes during recessions when the E collapses faster than the P. The long-run cyclically adjusted P/E (CAPE), which averages ten years of inflation-adjusted earnings, is also around 17 historically. The market has traded substantially above those long-run averages for most of the 2010s and 2020s, reflecting low interest rates, higher tech-sector weight, and accelerating share buybacks.

When trailing EPS is zero or negative, P/E is mathematically undefined or negative, and a negative P/E carries no useful interpretation. Data providers like Bloomberg, FactSet, and Yahoo Finance display N/A or a blank field for any loss-making company. This is common for young growth companies (Tesla had no meaningful trailing P/E for most of its first decade as a public company), biotech firms before drug approval, companies absorbing a one-time write-down, and any business in a cyclical trough. For unprofitable firms, analysts substitute price-to-sales, EV/EBITDA, EV/revenue, or price-to-free-cash-flow ratios to value the equity until earnings turn positive.

PEG divides trailing P/E by the expected annual earnings growth rate in percent. Peter Lynch popularized the rule in his 1989 book One Up on Wall Street: a PEG near 1.0 suggests the stock is fairly valued, below 1.0 looks cheap relative to its growth, and above 1.0 looks expensive. The bigger issue with P/E itself is that the multiple is not directly comparable across sectors and stages of the business cycle. Cyclicals (autos, banks, materials) print sky-high P/Es at the bottom of an earnings cycle and tiny P/Es at the peak, the exact opposite of what intuition suggests. Accounting choices (stock-based compensation, goodwill impairments, one-time tax items) can also distort GAAP EPS by 20 percent or more in a given year. Always cross-check P/E against EV/EBITDA, free cash flow yield, and the company's own historical multiple range before drawing conclusions.