How to use the P/E ratio

Introduction

The price-to-earnings ratio, usually shortened to P/E ratio, is one of the most common ways to describe a stock’s valuation. It compares the price investors pay for a share with the earnings attributable to that share.

The calculation is simple, but interpreting it requires context. A P/E of 10 is not automatically cheap, and a P/E of 40 is not automatically expensive. Growth, business quality, cyclicality, debt, interest rates, and the sustainability of earnings can all affect the multiple investors are willing to pay.

This guide explains how to calculate the P/E ratio, what it can tell investors, where it can be misleading, and how to use it as part of a broader stock valuation.

What is the P/E ratio?

The P/E ratio compares a company’s share price with its earnings per share, or EPS. It can be written as:

P/E ratio = Share price ÷ Earnings per share

The same ratio can also be calculated at the company level by dividing market capitalization by net income attributable to common shareholders, provided the numerator and denominator are measured consistently.

If a stock trades at $60 and generated $3 in EPS over the relevant period, its P/E ratio is 20. In simple terms, investors are paying $20 for each $1 of earnings represented by one share.

How to calculate the P/E ratio

Suppose a company reports net income of $500 million and has 100 million diluted shares outstanding. Its diluted EPS is $5. If the shares trade at $100, the P/E ratio is:

$100 ÷ $5 = 20

The company therefore trades at 20 times the earnings used in the calculation.

Investors should check which EPS figure a data provider uses. Basic EPS and diluted EPS can differ, and adjusted earnings may exclude items that remain included under GAAP. Consistency matters when comparing companies.

Trailing P/E versus forward P/E

A trailing P/E usually uses earnings reported over the most recent 12 months. A forward P/E uses estimated earnings for a future period, often the next 12 months or next fiscal year.

Neither measure is inherently superior. Trailing P/E is grounded in known results, while forward P/E may be more relevant when earnings are changing quickly. Our guide to the forward P/E ratio examines the forecast-based version in more detail.

What does a high P/E ratio mean?

A high P/E means investors are paying more for each dollar of current or expected earnings. This can happen for several reasons.

The market may expect faster future earnings growth. Investors may also assign a premium to companies with strong competitive positions, recurring revenue, high returns on capital, resilient margins, or relatively predictable earnings.

A high P/E can also reflect excessive optimism. If a stock’s valuation assumes rapid growth for many years, even a good business can disappoint investors if actual results fall short of those expectations.

What does a low P/E ratio mean?

A low P/E means investors pay less for each dollar of earnings. That can indicate an undervalued stock, but it can also reflect genuine concerns.

For example, earnings may be close to a cyclical peak, the company may have substantial debt, an important product may be declining, or investors may expect profits to fall. A one-time gain can also temporarily increase EPS and make the P/E appear unusually low.

This is why low-P/E stocks require the same business analysis as high-P/E stocks. The multiple tells investors what the market is paying, not whether the market is wrong.

What is a good P/E ratio?

There is no universal P/E ratio that defines a good valuation. A useful P/E depends on the company, industry, economic environment, and expected future performance.

A mature utility and a rapidly growing software company can reasonably trade at very different multiples. Even within one industry, differences in margins, balance-sheet strength, management quality, and growth can justify different valuations.

Instead of using a fixed threshold, investors can compare a company’s P/E with several relevant reference points.

• Its own history: Is the stock trading above or below its typical valuation, and has the business changed?
• Direct competitors: Are growth, profitability, and risk sufficiently similar for the comparison to be meaningful?
• The industry: Does the company deserve a premium or discount relative to the group?
• Expected growth: Are higher earnings growth expectations supporting a higher multiple?

Example: comparing two companies

Assume Company A trades at $80 with EPS of $4, giving it a P/E of 20. Company B trades at $90 with EPS of $3, giving it a P/E of 30.

Company A is cheaper based on current earnings. Company B, however, is expected to grow faster. An investor would need to decide whether that faster growth, along with any differences in risk and business quality, justifies paying the higher multiple.

The P/E ratio identifies the valuation difference. It does not by itself determine which stock offers better value.

Why earnings quality matters

The denominator of the P/E ratio is accounting earnings, so the quality and sustainability of those earnings matter. Net income can be affected by noncash charges, asset sales, impairments, tax items, restructuring costs, and accounting estimates.

Investors should look at the income statement and cash flow statement to understand whether reported profit is supported by the underlying business. Our guide to free cash flow explains an additional measure that can help evaluate how accounting profit translates into cash.

Adjusted EPS can sometimes provide useful context, but investors should examine what management excludes. Repeatedly excluding recurring expenses can make adjusted earnings look stronger than the economics of the business.

P/E ratios and cyclical companies

P/E ratios can be counterintuitive for cyclical businesses. Near the top of a cycle, earnings may be unusually high, which pushes the P/E ratio down. Near the bottom, earnings may collapse and the P/E can become very high or meaningless.

A low P/E at peak earnings can therefore make a cyclical stock look cheapest precisely when profits are least sustainable. Investors may instead examine normalized earnings across a full cycle and consider industry-specific measures.

When the P/E ratio does not work

The P/E ratio becomes difficult or impossible to interpret when a company has negative earnings. Dividing price by a loss does not produce a useful valuation multiple.

It can also be less informative when earnings fluctuate dramatically, contain large one-time items, or differ substantially from cash generation. In those cases, another valuation method may be more appropriate.

For businesses with meaningful differences in debt, EV/EBITDA can sometimes provide a more comparable view because enterprise value incorporates debt and cash. Asset-based businesses may also be evaluated using the price-to-book ratio.

P/E versus earnings yield

The earnings yield expresses the same relationship from the opposite direction. It divides EPS by share price instead of price by EPS.

Earnings yield = EPS ÷ Share price

A stock with a P/E of 20 has an earnings yield of 5% because 1 ÷ 20 equals 5%. This does not mean shareholders receive a 5% cash return. Earnings can be retained, reinvested, used for acquisitions, spent on buybacks, or distributed as dividends.

The earnings yield can nevertheless make valuation comparisons more intuitive, particularly when considering the return investors require for taking equity risk.

How growth affects the P/E ratio

Companies expected to grow earnings faster often trade at higher P/E ratios. Future growth can make today’s earnings a relatively small base compared with what the business may earn several years later.

However, growth should not be evaluated in isolation. Investors should consider how much capital the company needs to produce that growth and whether the expected rate can persist. The PEG ratio is one attempt to relate a P/E multiple directly to expected earnings growth.

A practical way to use the P/E ratio

1. Confirm the earnings figure. Determine whether the ratio uses trailing, forward, diluted, GAAP, or adjusted EPS.
2. Check for unusual items. Identify gains, losses, or cyclical conditions that may distort earnings.
3. Compare similar businesses. Peer comparisons work best when growth, margins, and risks are reasonably comparable.
4. Review historical valuation. Understand why the company’s multiple has changed over time.
5. Consider growth and quality. Higher multiples can be justified by better economics, but only to a point.
6. Cross-check another method. Use cash flow, enterprise-value multiples, or a DCF where appropriate.

Common P/E ratio mistakes

• Calling every low-P/E stock cheap.
• Comparing companies from very different industries.
• Ignoring whether earnings are at a cyclical peak or trough.
• Mixing trailing P/E for one company with forward P/E for another.
• Ignoring dilution and the difference between basic and diluted EPS.
• Assuming analysts’ future earnings estimates are certain.
• Using P/E for companies with losses.
• Looking at the multiple without examining debt, cash flow, and business quality.

Key takeaways

• The P/E ratio divides share price by earnings per share.
• It shows how much investors pay for each dollar of earnings represented by a share.
• A high P/E can reflect growth and quality, while a low P/E can reflect risk or declining earnings.
• There is no universally good P/E ratio.
• Trailing P/E uses reported earnings; forward P/E uses forecasts.
• P/E is most useful for profitable companies with reasonably meaningful earnings.
• Investors should compare multiples in context and use other valuation methods as cross-checks.