P/E Ratio Explained: How to Find Undervalued Stocks

The P/E ratio is the most widely quoted valuation metric in finance. You'll see it on every financial site, hear it on CNBC, and find it in every earnings report writeup. But most investors don't fully understand what it means — or how to use it correctly.

This guide breaks it down, shows you what to watch out for, and explains how to use the P/E ratio to spot undervalued stocks.

What Is the P/E Ratio?

P/E stands for Price-to-Earnings.

The formula is simple:

P/E Ratio = Stock Price ÷ Earnings Per Share (EPS)

If a stock trades at $50 and earns $5 per share, its P/E ratio is 10. That means you're paying $10 for every $1 of earnings.

A high P/E means investors are paying a premium — usually because they expect strong future growth. A low P/E may indicate the stock is cheap, or it may signal that the business is struggling.

Context is everything.

Trailing P/E vs. Forward P/E

You'll encounter two versions of the P/E ratio:

  • Trailing P/E (TTM) — Uses the last 12 months of actual earnings. More reliable because it's based on reported data.
  • Forward P/E — Uses analyst estimates for the next 12 months of earnings. More forward-looking, but subject to forecast error.

Both are useful. Trailing P/E tells you what you're paying for what the company has already produced. Forward P/E reflects expectations. If a stock's forward P/E is much lower than its trailing P/E, analysts expect earnings to improve significantly.

What Is a "Good" P/E Ratio?

There's no universal answer — it depends on the sector, growth rate, and market conditions.

General benchmarks:

  • S&P 500 historical average — roughly 15–17x earnings
  • Growth stocks — can trade at 30x, 50x, or higher if earnings growth justifies it
  • Value stocks — typically 8–12x, sometimes lower
  • Utilities and REITs — often use different metrics; P/E is less useful here

The right way to use the P/E ratio is comparatively:

  1. Compare to the company's own historical P/E range
  2. Compare to direct industry peers
  3. Compare to the broader market

A P/E of 20 might be cheap for a software company and expensive for a grocery chain.

The PEG Ratio: P/E Adjusted for Growth

A common criticism of the P/E ratio is that it ignores growth. A stock at 25x earnings looks expensive — unless it's growing earnings at 40% per year.

That's where the PEG ratio comes in:

PEG = P/E Ratio ÷ Earnings Growth Rate

A PEG below 1.0 is often considered undervalued. A PEG above 2.0 suggests the market may be pricing in too much optimism.

Invented by Peter Lynch, the PEG ratio is a quick way to normalize the P/E for growth.

Limitations of the P/E Ratio

Used carelessly, P/E can mislead you. Here's what to watch out for:

  • Earnings manipulation — Net income can be influenced by accounting choices. A company can look cheaper than it really is if earnings are inflated.
  • One-time items — A large asset sale or tax benefit can artificially boost EPS. Always look at adjusted or normalized earnings.
  • Cyclical stocks — At the peak of a cycle, earnings are high, so P/E looks low. At the trough, losses make P/E look misleading or even negative.
  • Negative earnings — P/E doesn't work for unprofitable companies. Use EV/EBITDA or Price/Sales instead.

For a more complete picture, pair the P/E ratio with free cash flow analysis and a full valuation model like a DCF.

How to Use the P/E Ratio to Screen for Undervalued Stocks

Here's a practical process:

  1. Set a P/E filter — Use a stock screener to find companies trading below the sector median P/E.
  2. Check earnings quality — Make sure EPS is backed by real cash flow, not accounting adjustments.
  3. Look at the trend — Is the P/E declining because earnings are rising, or because the price is falling?
  4. Dig deeper — A low P/E alone doesn't make a stock a buy. It's a starting point for further analysis.

Elite Stock Research's stock screener lets you filter US stocks by P/E ratio, P/B, dividend yield, market cap, and more — completely free.

The Bottom Line

The P/E ratio is one of the most useful — and most misused — tools in an investor's arsenal. Used correctly, in context, alongside other metrics, it's a powerful shortcut for spotting potential value.

But it's just the beginning. Cheap stocks get cheaper for a reason. Do the full work.

→ Screen stocks by P/E ratio for free at elitestockresearch.com