The P/E Ratio Explained: How to Tell If a Stock Is Cheap or Expensive

Turn on any financial news channel and you'll hear it within minutes: "The stock trades at 25 times earnings." "The S&P 500's forward P/E is above its long-term average." The price-to-earnings (P/E) ratio is the most widely used valuation measure in the US stock market. It's simple, it's everywhere, and it's genuinely useful — as long as you understand what it does and doesn't tell you.

In this guide we'll explain exactly how the P/E ratio works, the different versions you'll encounter, how to interpret high and low P/Es, how to compare companies fairly, and the common mistakes that lead investors astray.

What Is the P/E Ratio?

The P/E ratio compares a company's share price with its earnings per share (EPS):

P/E = Share price ÷ 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. In plain English, investors are paying $20 for every $1 of annual profit.

Another way to think about it: at a P/E of 20, if earnings stayed flat forever, it would take 20 years of profits to "pay back" the price of the stock. A higher P/E means investors expect earnings to grow; a lower P/E suggests lower expectations, higher perceived risk, or a potential bargain.

Trailing vs. Forward P/E

You'll encounter two main versions:

• Trailing P/E (TTM): Uses actual reported earnings from the last twelve months. It's based on facts, but it looks backward.
• Forward P/E: Uses analysts' estimated earnings for the next twelve months. It looks ahead, but depends on forecasts that may prove wrong.

When a company's earnings are growing quickly, its forward P/E will be noticeably lower than its trailing P/E. When earnings are expected to fall, the forward P/E will be higher. Comparing the two quickly tells you what the market expects.

The Earnings Yield: The P/E Turned Upside Down

Flip the P/E and you get the earnings yield: EPS divided by price. A P/E of 20 corresponds to an earnings yield of 5%. This is useful for comparing stocks with bonds. If 10-year Treasury bonds yield more than the stock market's earnings yield, stocks look relatively less attractive — one reason why Federal Reserve policy and interest rates have such a strong influence on valuations.

What Is a "Good" P/E Ratio?

There's no universal answer. A P/E of 30 might be cheap for a company growing earnings 40% a year and expensive for a utility growing 3%. Context is everything. Professionals use three types of comparison:

1. Against the company's own history. Is the stock trading above or below its average P/E over the past five or ten years? A big deviation invites the question "what's changed?"
2. Against industry peers. Software companies, banks and oil producers naturally trade at very different multiples. Compare like with like.
3. Against the overall market. The S&P 500's P/E has historically averaged in the mid-teens over the very long term, though it has spent much of the past few decades above that level. You can follow index data at S&P Dow Jones Indices.

Why Do Some Stocks Have High P/E Ratios?

High P/Es usually reflect one or more of the following:

• Rapid expected growth. Investors pay up today for much higher earnings tomorrow — the classic case for growth stocks.
• High quality and stability. Companies with dominant brands, recurring revenue and strong balance sheets often command premium multiples.
• Temporarily depressed earnings. If profits fall sharply because of a one-time event, the P/E can spike even though the business is fine.
• Low interest rates, which raise the value of future earnings across the market.
• Speculation and hype, which can push valuations well beyond fundamentals.

Why Do Some Stocks Have Low P/E Ratios?

• Slow or no growth. Mature businesses in stable industries often trade at modest multiples.
• Cyclicality. Companies in industries like autos, steel or energy often look cheapest at the peak of the cycle, when earnings are temporarily high.
• Elevated risk. High debt, legal problems, regulatory threats or disruption fears push P/Es down.
• Genuine undervaluation. Sometimes the market is simply too pessimistic — the opportunity value investors hunt for.

The PEG Ratio: Adding Growth to the Equation

Because P/E ignores growth, many investors use the PEG ratio:

PEG = P/E ÷ Expected annual EPS growth rate (%)

A company with a P/E of 30 and expected growth of 30% has a PEG of 1.0. A company with a P/E of 15 and expected growth of 5% has a PEG of 3.0. Despite its lower P/E, the second company may actually be more expensive relative to its growth. Peter Lynch popularized the rule of thumb that a PEG around 1 suggests a fair price, though growth estimates are inherently uncertain.

Common P/E Ratio Traps

1. The value trap A low P/E can signal a business in decline. If earnings are about to fall, today's "cheap" P/E is an illusion. Always ask why the stock is cheap.

2. The cyclical trap For cyclical companies, P/Es often look lowest when earnings are at a peak — just before they fall. Experienced investors sometimes buy cyclicals when P/Es look high because earnings are temporarily depressed.

3. Negative or tiny earnings If a company loses money, the P/E is meaningless. If earnings are barely positive, the P/E can be absurdly high. In these cases, use price-to-sales, EV/EBITDA or price-to-free-cash-flow instead.

4. Adjusted vs. GAAP earnings A P/E based on "adjusted" earnings can look much more attractive than one based on GAAP earnings. Know which version you're seeing. Our guide to reading an income statement explains the difference.

5. Share buybacks Buybacks reduce the share count, boosting EPS even if total profits are flat. That's not necessarily bad, but it's important to know where EPS growth is coming from.

6. Ignoring debt P/E looks only at equity. Two companies with the same P/E can have very different risk profiles if one carries heavy debt. Enterprise-value-based metrics such as EV/EBIT account for this.

Other Valuation Metrics to Use Alongside P/E

• Price-to-sales (P/S): Useful for unprofitable or early-stage companies.
• Price-to-book (P/B): Common for banks and insurers.
• EV/EBITDA: Compares the value of the whole business, including debt, with operating profit before non-cash charges.
• Price-to-free-cash-flow: Focuses on actual cash generation. See our free cash flow guide.
• Dividend yield: Relevant for income investors; see our dividend investing guide.

No single metric is enough. Combine P/E with business analysis from the company's 10-K report and our full stock analysis checklist.

A Step-by-Step Example: Comparing Two Stocks

Suppose you're comparing two retailers:

• Retailer X: Share price $80, trailing EPS $4.00, forward EPS $4.80, expected growth 20%.
• Retailer Y: Share price $60, trailing EPS $5.00, forward EPS $5.15, expected growth 3%.

Retailer X has a trailing P/E of 20 and a forward P/E of about 16.7. Retailer Y has a trailing P/E of 12 and a forward P/E of about 11.7. At first glance, Y looks much cheaper.

Now add growth. Retailer X's PEG is 20 ÷ 20 = 1.0. Retailer Y's PEG is 12 ÷ 3 = 4.0. Measured against growth, X looks more reasonably priced.

But the analysis doesn't stop there. You'd want to check whether X's 20% growth estimate is realistic, whether its margins are sustainable, how much debt each company carries, and whether Y pays a meaningful dividend that adds to its total return. This is exactly the kind of multi-factor thinking our stock analysis checklist is designed to support.

P/E Ratios Across Sectors

Typical valuation levels differ widely by sector because of differences in growth, risk and capital needs:

• Technology and software: Often above the market average because of high margins and growth.
• Consumer staples: Usually moderate premiums for stability and dividends.
• Utilities: Moderate multiples tied closely to interest rates.
• Financials: Often below the market, and frequently analyzed with price-to-book instead.
• Energy and materials: Highly variable because earnings swing with commodity prices.

That's why a "cheap" tech stock and a "cheap" bank stock can have completely different P/Es. Comparing within a sector — for example, using a sector ETF's average valuation as a reference — gives more meaningful results.

The Shiller CAPE Ratio

Economist Robert Shiller developed the cyclically adjusted P/E (CAPE), which divides the market's price by average inflation-adjusted earnings over the past ten years. Smoothing earnings over a full business cycle reduces distortions from booms and recessions. CAPE has been a useful gauge of long-term market valuation, though it's a poor short-term timing tool — markets can stay "expensive" for years.

Key Takeaways

• P/E = share price ÷ earnings per share; it shows how much investors pay for $1 of profit.
• Trailing P/E uses past earnings; forward P/E uses estimates.
• Compare P/Es with the company's history, its peers and the overall market.
• Use the PEG ratio to account for growth, and other metrics when earnings are negative or distorted.
• Beware value traps, cyclical traps and adjusted earnings that flatter the numbers.