Is a $500 stock expensive and a $5 stock cheap? Not necessarily. Share price alone says nothing about value. What matters is how much you pay relative to what the business earns, owns and is likely to earn in future. Valuation ratios make that comparison possible.
The price-to-earnings (P/E) ratio
The P/E ratio divides the share price by earnings per share (EPS). If a stock trades at $100 and earned $5 per share over the last year, its P/E is 20. You are paying $20 for every $1 of annual profit. Flip it around and you get the earnings yield: 1 ÷ 20 = 5%, a handy way to compare a stock with bond yields.
- Trailing P/E uses the last 12 months of actual earnings.
- Forward P/E uses analysts’ estimates of the next 12 months. It looks further ahead but depends on forecasts that can be wrong.
Why do P/E ratios differ so much?
A utility might trade at 15 times earnings while a fast-growing software company trades at 50. That is not necessarily irrational. Investors pay more for:
- Faster growth: future earnings will be much larger.
- Higher quality: strong margins, little debt, durable competitive advantages.
- Lower risk: stable, predictable profits.
- Lower interest rates: future profits are worth more in today’s money when rates are low.
That is why comparing P/E ratios only makes sense among similar companies, or against a company’s own history.
The PEG ratio: adjusting for growth
The PEG ratio divides the P/E by the expected annual earnings growth rate. A company with a P/E of 30 growing earnings 30% a year has a PEG of 1.0; one with a P/E of 15 growing 5% has a PEG of 3.0. By this measure, the “expensive” stock may be the better value. A PEG near or below 1 is often considered reasonable, but the growth estimate is the weak link.
Other valuation tools
| Metric | Formula | Best for |
|---|---|---|
| Price-to-sales (P/S) | Market cap ÷ revenue | Young or unprofitable companies |
| Price-to-book (P/B) | Price ÷ book value per share | Banks, insurers, asset-heavy firms |
| EV/EBITDA | Enterprise value ÷ EBITDA | Comparing companies with different debt levels |
| Free cash flow yield | Free cash flow ÷ market cap | Judging real cash returned to owners |
| Dividend yield | Annual dividend ÷ price | Income investors |
Enterprise value (EV) is market cap plus debt minus cash, the price to buy the whole business including its obligations. It gives a fairer comparison between a debt-free company and a heavily indebted one.
A simple fair-value estimate
One practical approach combines growth and P/E:
- Start with current EPS, say $6.50.
- Project growth, say 10% a year for five years: EPS becomes about $10.47.
- Apply a reasonable exit P/E, say 22: the projected price is about $230.
- Discount back at your required return, say 9% a year: today’s fair value is about $150.
If the stock trades at $180, it is above this estimate; at $120 it offers a margin of safety. Our fair value calculator runs these steps instantly so you can test different assumptions. The answer is only as good as the inputs, so try a pessimistic case too.
Valuation traps to avoid
- The value trap: a low P/E can mean the market expects earnings to fall. Check why it is cheap.
- Cyclical peaks: carmakers, miners and chipmakers often look cheapest at the top of the cycle, when earnings are temporarily high.
- One-off items: asset sales or write-downs distort EPS. Look at adjusted or operating earnings and cash flow.
- Share dilution: heavy stock-based compensation increases share count and quietly reduces your slice.
- Ignoring debt: a low P/E with a mountain of debt can be riskier than it looks. Use EV-based metrics.
Valuation and timing
Valuation is a poor short-term timing tool. Expensive stocks can get more expensive for years, and cheap ones can stay cheap. Over long horizons, however, the price you pay strongly influences your returns. Use valuation to decide what deserves a place on your watchlist, then use charts and risk rules to manage entries.
Key takeaways
- P/E shows what you pay per dollar of earnings; compare like with like.
- Growth, quality, risk and interest rates justify different multiples.
- Use several metrics, including cash flow and EV-based ones.
- Always ask why a stock is cheap before calling it a bargain.
Educational content only, not investment advice.