What is a good PE ratio?

There is no universal 'good' PE ratio. Historically, the S&P 500 has averaged a PE in the 15 to 18 range, so anything meaningfully below that is often considered cheap and anything meaningfully above is considered expensive. But context matters enormously: a fast-growing tech company at 30x can be a better deal than a no-growth utility at 12x.

What is the formula for PE ratio?

PE ratio equals share price divided by earnings per share (EPS). If a stock trades at $200 and earned $10 per share over the last 12 months, its trailing PE is 20. You can also calculate it at the company level by dividing market capitalization by net income — the answer is the same.

What is the difference between trailing and forward PE?

Trailing PE uses earnings from the last 12 months — a real, reported number. Forward PE uses analyst estimates of earnings over the next 12 months — a projection. Forward PE is almost always lower because analysts assume growth, and it's almost always wrong by some amount because the future doesn't follow forecasts.

Can a PE ratio be negative?

Yes, when a company has negative earnings (a loss). Most data providers display 'n/a' instead of a negative number because a negative PE has no useful interpretation — you can't be 'paying -$15 for $1 of losses' in any meaningful sense. For unprofitable companies, investors use other metrics like price-to-sales or EV/EBITDA.

Why do tech stocks have higher PE ratios?

Because investors expect their earnings to grow much faster than average. A PE of 40 is 'cheap' if a company will double its earnings in three years — the high multiple gets absorbed by the growth. The same PE on a no-growth company is genuinely expensive. Sector PEs reflect baseline growth expectations for the industry.

What Is a PE Ratio? A Plain-English Explanation

If you've ever read a stock analysis, you've seen the PE ratio. NVDA trades at 65x. Apple at 32x. Coca-Cola at 25x. These numbers get thrown around constantly, usually with the implicit assumption that you already know what they mean. Most people don't, exactly — they have a vague sense that lower is cheaper and higher is expensive, and they leave it at that.

That vague sense is roughly right and completely useless. Let's actually unpack it.

The math, in one sentence

The PE ratio is what you pay for every dollar of a company's annual earnings. Stock price ÷ earnings per share = PE ratio. If Apple trades at $192 and earned $6 per share over the last 12 months, its PE is 32. That means: for every $1 of profit Apple generates per year, you're paying $32 of stock price.

You can flip the same math at the company level: market capitalization divided by net income gives you the same answer. A company worth $3 trillion that earned $100 billion last year has a PE of 30. The per-share version and the company-level version always agree, because they're the same ratio scaled differently.

What it actually tells you

Here's the useful mental model. Forget "ratio" for a second and think of the PE as a payback period. A PE of 20 means: if the company's earnings stayed exactly flat forever and they paid you all of those earnings every year, you'd get your money back in 20 years. A PE of 10 means 10 years. A PE of 50 means 50 years.

Now you can see why a high PE isn't automatically bad and a low PE isn't automatically good. A PE of 50 is fine if you genuinely believe earnings will quintuple over the next decade — the payback period collapses fast when earnings grow. A PE of 8 is terrible if earnings are about to fall off a cliff. The number itself doesn't decide; your forecast of future earnings does.

The PE ratio is a question, not an answer. It asks: do you believe this company's earnings will justify what you're paying?

Why people obsess over it

Three reasons. First, it's easy. You can calculate it in your head. No model, no spreadsheet, no assumptions. Second, it's universal — every public company that has earnings has a PE, so you can compare across the entire market in seconds. Third, decades of academic research have shown that, on average, lower-PE stocks have outperformed higher-PE stocks over long periods. This is the entire foundation of "value investing" as a discipline.

The catch is that "on average over long periods" doesn't help you with the specific stock in front of you today. Every cheap stock looks like a value trap until it isn't, and every expensive stock looks justified until it isn't. The PE tells you what the market is asking. It doesn't tell you whether the answer is right.

Try it on a real stock

The PE Sanity Check tells you if a given PE is justified by growth and the risk-free rate.

Open the PE Sanity Check →

The gotchas nobody mentions

There are at least four things about PE ratios that get glossed over in most explanations.

Trailing vs forward. Trailing PE uses earnings from the last 12 months — a reported, real number. Forward PE uses analyst estimates of next year's earnings — a guess. Forward PE is almost always lower (because analysts assume growth) and almost always wrong by some amount. When someone quotes a "PE of 22," ask which one.
Negative earnings break it. If a company lost money, its PE is mathematically negative or undefined. Most data providers show "n/a." A loss-making company isn't "PE 0" or "PE -15" — it's a company where the PE framework simply doesn't apply, and you need a different lens (price-to-sales, EV/EBITDA, free cash flow yield).
Earnings can be manipulated. Net income is an accounting construct. Companies can take one-time charges, change depreciation schedules, capitalize expenses, or buy back stock to flatter the per-share number. A PE that drops from 25 to 18 on the back of a one-time accounting change isn't actually any cheaper.
Sector context is everything. Comparing Coca-Cola's PE to NVIDIA's PE is meaningless. Beverages and semiconductors have different growth rates, different capital intensity, and different baseline expectations. The right comparison is always within the same sector — Coke vs Pepsi, NVIDIA vs AMD.

When PE breaks down completely

There are categories of company where the PE ratio is essentially worthless and you should not use it at all:

Loss-making growth companies. Pre-profit startups, biotech firms in clinical trials, early-stage SaaS — all have negative or no earnings. The PE is undefined.
Cyclical companies at peak earnings. A homebuilder at the top of a housing cycle might trade at PE 6 — and that's not cheap, that's a warning. Earnings are about to fall and the "low" PE is an artifact of the peak.
Holding companies and conglomerates. Berkshire Hathaway's earnings are dominated by mark-to-market gains on its stock portfolio, which can swing by tens of billions per quarter. Its PE bounces around meaninglessly.
Companies in major transitions. A firm that just sold a division, completed a huge acquisition, or took a massive write-down has earnings that don't reflect its actual ongoing economics.

How to use it well

Treat the PE as the start of a question, not the end of an analysis. When you see a stock at PE 35, the right next question isn't "is 35 too high?" — it's "what growth rate is implied by 35, and is that growth rate plausible?" If the company needs to grow earnings 25% per year for five years to justify the multiple, ask whether that's actually likely. If the answer is yes, 35 might be cheap. If the answer is no, 35 is a problem.

That's exactly the math the PE Sanity Check runs for you. You plug in the PE, the growth rate you expect, and your country's risk-free rate. It tells you whether the implied return clears the hurdle of doing nothing safer instead. It won't tell you whether your growth forecast is right — nobody can do that — but it will tell you whether the math holds up if you are.

The PE ratio survived a hundred years of investing because it captures something real. Used carelessly, it's a recipe for buying value traps and missing growth winners. Used thoughtfully, it's the cleanest single number for asking "is this stock priced for a future that's actually going to happen?"

What Is a PE Ratio?