How to Calculate the PEG Ratio for Better Stock Picks

The PEG Ratio Adds the One Dimension P/E Ignores

A P/E ratio of 28 means nothing without knowing how fast earnings are growing. Two companies can both trade at a P/E of 28. One grows earnings at 5% per year. The other grows at 28% per year. They are not the same investment. The PEG ratio separates them in a single calculation.

Peter Lynch popularized the PEG ratio in his 1989 book "One Up on Wall Street." His rule: a PEG of 1.0 means fair value. Below 1.0, the stock deserves a second look. Above 2.0, the growth premium has likely exceeded what future earnings can support.

The Formula: Plain and Exact

PEG Ratio = P/E Ratio / EPS Growth Rate

The EPS growth rate is expressed as a whole number, not a decimal. A 15% growth rate enters the formula as 15, not 0.15.

Trailing PEG uses the last twelve months of earnings per share and historical EPS growth.

Forward PEG uses next-twelve-month earnings estimates and projected growth rates sourced from analyst consensus or company guidance.

Both versions are valid. Forward PEG is more predictive. Trailing PEG is more auditable. Use forward PEG for growth stocks, where the current earnings base understates the trajectory. Use trailing PEG for mature businesses, where the historical record is more reliable than analyst projections.

Worked Example 1: Tech Stock at a 30 P/E

A software company trades at $180 per share. It earned $6.00 per share over the trailing twelve months. That produces a trailing P/E of 30.

EPS grew from $3.75 three years ago to $6.00 today. Annualized, that growth rate is approximately 17%.

PEG = 30 / 17 = 1.76

At 1.76, the stock carries a meaningful growth premium but sits below the 2.0 danger threshold Lynch identified. An investor comparing this to a second software company at a P/E of 22 growing EPS at 9% per year would find:

PEG = 22 / 9 = 2.44

The second company looks cheaper on P/E. Its PEG tells a different story. You pay 2.44x the growth rate per unit of earnings, versus 1.76x for the first. The higher-P/E stock is the better-valued growth investment by this measure.

Worked Example 2: Consumer Staples Stock at a "Low" P/E

A consumer staples company trades at $95 per share with trailing EPS of $5.28, producing a P/E of roughly 18. Compared to the S&P 500's long-run average P/E near 16, this looks modestly valued.

EPS three years ago was $4.89. Annualized growth is approximately 2.6%. Round to 3 for the formula.

PEG = 18 / 3 = 6.0

A PEG of 6.0 is not a screaming buy at any price. The 18x earnings multiple assumes growth that simply is not present. An investor who bought based on P/E alone would carry an asset priced for performance the company cannot deliver.

This is the structural overpayment that P/E misses and PEG surfaces immediately.

Three Inputs That Break the PEG Ratio

Negative EPS Growth Renders PEG Meaningless

A company with declining earnings produces a negative PEG. The formula returns a number, but that number carries no interpretive value. PEG applies only to companies with positive and sustained EPS growth. For turnaround plays or cyclical bottoms, use price-to-book or EV/EBITDA instead.

Artificially Low Earnings Distort the P/E Input

A company that took a large one-time write-off in the trailing twelve months will show artificially low EPS and an inflated P/E. That inflated P/E flows directly into an inflated PEG. Always check whether EPS has been distorted by non-recurring charges before trusting the ratio.

Analyst Growth Estimates Carry Systematic Bias

Forward PEG depends on analyst EPS growth estimates. Research from CFA Institute and academic finance consistently shows sell-side analysts overestimate forward earnings growth by an average of 3 to 5 percentage points. A stock with a consensus 25% forward growth estimate and a P/E of 40 looks fairly valued at PEG = 1.6. Adjust the growth rate down to a more conservative 20%, and PEG = 2.0. That 5-point difference materially changes the conclusion. Apply a 15% to 20% haircut to consensus growth estimates before running the PEG calculation on high-expectation stocks.

PEG as a Screening Tool, Not a Final Verdict

The PEG ratio screens efficiently. It eliminates obviously overpriced growth stocks and identifies potentially undervalued ones in minutes. It does not replace analysis of balance sheet quality, competitive positioning, management capital allocation, or sector-level risks.

Use PEG to narrow a list of 40 candidates to 8 worth deeper work. Do not use it to approve a final buy decision on its own.

A PEG below 1.0 on a company with a net debt-to-EBITDA ratio above 5.0x, deteriorating gross margins, and heavy insider selling is not a buying signal. The PEG ratio asked a question about growth-adjusted valuation. The other data answered it differently.

Run Your Stocks Through the CalcMoney Investment Calculator

The PEG formula is simple arithmetic. The judgment around which inputs to use, which growth rate to trust, and how to compare across sectors is where the analysis lives. The CalcMoney Investment Calculator lets you model forward scenarios with adjustable growth rate assumptions, stress-test valuations against conservative and aggressive EPS projections, and compare multiple positions side by side.

Before you buy at a P/E of 32 because "the sector always trades at a premium," run the PEG. The number takes thirty seconds to produce. The information it returns is worth considerably more than that.

Open the CalcMoney Investment Calculator and run your PEG analysis now →

You Might Also Like

• How to Calculate Year-Over-Year Growth Rate for Your Investment Portfolio
• How to Calculate Dividend Growth Rate (And Why Most Income Investors Get It Wrong)
• How to Calculate Earnings Per Share and Use It to Value Stocks

Results are estimates for informational purposes only. Consult a licensed financial professional before making financial decisions.