What Is the PEG Ratio? Price/Earnings-to-Growth Explained

What is the PEG ratio?

PEG = P/E Ratio ÷ Annual EPS Growth Rate (%)

The PEG ratio solves a core problem with the P/E ratio: a company growing earnings at 40% per year deserves a higher P/E than one growing at 5% — but P/E alone doesn't tell you whether you're paying fairly for that growth.

Peter Lynch, the legendary Fidelity fund manager who averaged 29% annual returns in the Magellan Fund, popularised PEG in his book One Up on Wall Street (1989). His rule of thumb: a "fairly valued" stock has a PEG of 1.0 — P/E equal to the growth rate.

How to apply PEG in practice

Example: Company A has a P/E of 25 and is expected to grow EPS at 25% per year → PEG = 1.0 (fairly valued). Company B has a P/E of 25 but is only growing at 10% → PEG = 2.5 (potentially overvalued). Company C has a P/E of 15 and growing at 20% → PEG = 0.75 (potentially undervalued).

When you find a stock with a PEG below 1.0, it suggests the market is not fully pricing in the expected growth rate. Lynch called these "fast growers selling at a discount."

Growth rate: which one to use?

Forward growth: Consensus analyst EPS growth estimate for the next 3–5 years. More relevant but depends on forecast accuracy.

Historical growth: Trailing 3–5 year actual EPS CAGR. More reliable but may not reflect future potential.

Blended: Some analysts average historical and forward to reduce the impact of optimistic estimates.

PEG limitations

The PEG ratio is only as good as the growth estimate. Analysts systematically overestimate long-run growth, which means PEGs often look more attractive than they truly are. A 30% expected growth rate that materialises at 15% means a PEG of 0.8 was actually 1.6 all along.

PEG also breaks down for very low-growth or no-growth businesses — a dividend utility growing at 2% with a P/E of 15 has a PEG of 7.5, which looks terrible but is perfectly appropriate for that business type.

Key takeaways

PEG = P/E ÷ growth rate. Adjusts P/E for the value of future earnings growth.
PEG below 1.0: potentially undervalued relative to growth. Above 2.0: potentially expensive.
Growth rate input critically affects the result — use conservative estimates.
Most useful for growth companies; less meaningful for mature or declining businesses.

Frequently Asked Questions

What is the PEG ratio formula?

PEG = P/E Ratio ÷ Expected Earnings Growth Rate (%). A stock with a P/E of 20 and expected earnings growth of 20% per year has a PEG of 1.0. Peter Lynch's rule of thumb: PEG below 1.0 suggests the stock is undervalued relative to its growth; above 2.0 suggests overvaluation.

What is a good PEG ratio?

Peter Lynch considered PEG below 1.0 as potentially undervalued and above 2.0 as expensive. However, these thresholds vary by interest rate environment. In a low-rate world, investors accept higher PEGs. Always compare PEG to the stock's own history and sector peers.

What growth rate do I use for PEG?

Most investors use the consensus analyst estimate for the next 3–5 years of EPS growth. Some use the trailing 3–5 year actual growth rate as a more conservative estimate. The choice significantly affects the output — always be consistent in what you use.

What are the limitations of the PEG ratio?

PEG depends entirely on the accuracy of the growth estimate. If a company misses its growth targets, a seemingly cheap PEG becomes expensive quickly. Also, PEG is meaningless for companies with negative earnings or very low growth rates.