The World's Most Popular Valuation Metric

The Price-to-Earnings (P/E) ratio is the most widely used metric for valuing stocks. At its simplest: P/E = Stock Price ÷ Earnings Per Share (EPS). It tells you how much investors are willing to pay for each dollar of a company's earnings.

If a stock trades at $100 and earned $5 per share over the last year, its P/E is 20. That means investors are paying $20 for every $1 of earnings. Whether that's cheap or expensive depends on context.

Types of P/E Ratios

Trailing P/E — Uses the last 12 months of actual reported earnings. Most commonly quoted. Based on facts, not forecasts.
Forward P/E — Uses analyst estimates for the next 12 months of earnings. Optimistic — analysts tend to overestimate. But forward P/E is what professionals use because the market prices future earnings, not past earnings.
Shiller P/E (CAPE) — Uses average inflation-adjusted earnings over the past 10 years. Created by Nobel laureate Robert Shiller. Useful for assessing whether the overall market is expensive relative to history.

What's a "Good" P/E?

There's no universal answer. A P/E of 15 might be expensive for a slow-growing utility but dirt-cheap for a tech company growing earnings at 40% annually. Context matters:

Compare to the company's own history — Is its current P/E above or below its 5-year average?
Compare to peers — What's the median P/E for the industry? A 25 P/E might be reasonable if competitors trade at 35.
Compare to growth rate (PEG ratio) — P/E divided by earnings growth rate. A PEG below 1.0 is often considered undervalued.
Consider the interest rate environment — When interest rates are low, investors accept higher P/Es because bonds offer poor alternatives. When rates rise, P/Es tend to contract.

Limitations of P/E

Earnings can be manipulated — One-time gains, write-offs, and accounting changes can distort reported earnings. Always check if earnings are "adjusted" or "GAAP."
P/E is meaningless for unprofitable companies — If earnings are negative, P/E is negative or N/A. Use other metrics like Price-to-Sales or Price-to-Book for these companies.
Cyclical earnings distortion — For cyclical companies (autos, commodities, semiconductors), earnings swing wildly. P/E looks lowest at the peak of the cycle — exactly when you should be selling, not buying.

Key Takeaways

P/E = price ÷ earnings. It's a starting point for valuation, not a final answer.
Always compare P/E to historical averages, industry peers, and growth rates. Context is everything.
Forward P/E is more relevant than trailing P/E, but be aware that analyst estimates are often optimistic.