What Exactly Is the PEG Ratio and How Do You Calculate It?

Simply put, the PEG ratio helps answer one question: whether a stock is expensive depends on how fast it's growing. The formula is: PEG = P/E ÷ Expected Annual EPS Growth Rate.^[1]

For example: Company A has a P/E of 30 and expected earnings growth of 30%; Company B also has a P/E of 30 but only 10% growth. A's PEG = 30 ÷ 30 = 1, B's PEG = 30 ÷ 10 = 3. A looks cheaper because, at the same price, it's growing faster.

Watch out for a common mistake: When calculating, use the growth rate as a whole number (e.g., 30% as 30), not 0.3. Otherwise, the PEG will be inflated 100 times and completely wrong.^[2]

Why can't you use decimals? Because the P/E is a multiple, and using a whole number keeps the units consistent. If you write 30% as 0.3, then PEG = 30 ÷ 0.3 = 100, making a reasonably valued stock look ridiculously expensive. Many beginners trip here: always use the number, e.g., 25% as 25, not 0.25.

Is PEG = 1 Fair Value? Who Came Up With It?

The PEG ≈ 1 benchmark was first proposed by Mario Farina in 1969 and later popularized by legendary fund manager Peter Lynch in his 1989 book One Up on Wall Street.^[4] Lynch believed that a fairly valued company's P/E should roughly equal its growth rate—so PEG = 1.

Thus, PEG < 1 may mean the stock is undervalued relative to its growth, and PEG > 1 may mean the market is paying a premium for growth.^[3] But remember: this is a rule of thumb, not a precise buy signal.

Think of it like buying fruit: PEG = 1 means you're paying a fair price per unit of sweetness; PEG < 1 means you got sweet fruit at a bargain; PEG > 1 means you paid a lot for just a little sweetness. But you can taste fruit sweetness, while company growth is only a forecast—so this rule isn't foolproof.

What's the Difference Between PEG and P/E?

The P/E ratio is stock price divided by earnings per share. It only tells you "how much you pay for each dollar of earnings right now," completely ignoring future growth.^[12] For example, two companies both have a P/E of 30, but one grows 10% and the other 30%. Looking only at P/E, they seem equally expensive, but they're very different.

PEG fills this gap: it combines P/E and growth rate, letting you compare companies with different growth speeds on a level playing field. If you're not familiar with P/E, check out our article on What Is the P/E Ratio.

Here's a real-life analogy: P/E is like looking only at a house's total price without considering its size; PEG combines price and size to give you price per square meter. Two companies with the same P/E but different growth rates are like two houses with the same total price—one is 100 square meters, the other 50. Clearly, the 100-square-meter one is a better deal.

Should You Use Historical or Forecast Growth for PEG?

It depends on whether you want to look at the past or the future. Trailing PEG uses the historical growth rate over the past 3–5 years. Its advantage is that the data is real and not influenced by analyst sentiment, but it may miss companies that have just accelerated growth.^[7]

Forward PEG uses analysts' forecasts for the next 1–5 years. It reflects current market expectations, but forecasts can be inaccurate. In fact, a CFA Institute study shows that analysts' forecasts for 3–5 year earnings growth have historically averaged 10%–15% higher than actual results.^[6] This means a seemingly cheap PEG might just be due to overly optimistic forecasts.

For example: Suppose a company has a Forward PEG of 0.8, looking cheap. But if analysts predicted 20% growth and actual growth is only 10%, the real PEG becomes 1.6—actually expensive. So when using Forward PEG, always ask: Is this growth forecast reliable?

Also, different analysts may give very different growth forecasts—some optimistic, some conservative. This can cause the same stock's Forward PEG to vary across platforms. It's a good idea to compare at least two or three sources, or make your own judgment based on the company's history and industry outlook.

Is a Lower PEG Always Better? What Are the Pitfalls?

Not necessarily. PEG has several clear limitations: First, it ignores risk. Two companies both have a PEG of 1.0, but one has stable cash flow and the other is heavily in debt—PEG doesn't show this difference.^[8]

Second, it completely ignores dividends. For slow-growing but high-dividend utility stocks, PEG will look high, but that doesn't mean they're bad. A modified version, the PEGY ratio, adds the dividend yield to the denominator: PEGY = P/E ÷ (Expected Growth Rate % + Dividend Yield %). A PEGY < 1 is also seen as a possible undervaluation signal.^[9]

Third, PEG cannot be compared across industries. High-growth software stocks typically have higher PEGs than mature utility stocks because the market is willing to pay a premium for more certain growth.^[11] So PEG is best used to compare companies within the same industry and business model.

Additionally, PEG's reliability as a timing tool is questionable. A CFA Institute study notes that PEG falling below 1 is historically rare at the market level, especially after 2000, and using PEG as a market-timing signal has not been stable.^[6] In other words, don't rely on PEG to predict market moves.

What Is the GARP Strategy and How Does It Relate to PEG?

GARP stands for "Growth At a Reasonable Price." It's a stock-picking strategy that combines growth and value investing.^[10] Investors often use PEG to screen targets, with PEG ≈ 1 or below considered attractive.

This strategy became well-known through Peter Lynch's management of the Fidelity Magellan Fund. Simply put, GARP investors don't want to buy "value traps" with no growth, nor do they want to pay excessive premiums for high growth—PEG is their measuring stick.

For example: A company with a P/E of 20 and growth of 20% has a PEG of 1, meeting GARP criteria. Another with a P/E of 40 and growth of 30% has a PEG of 1.33, which may be too expensive. A GARP investor would prefer the former because it offers decent growth at a reasonable price.

Where Can Ordinary Investors Find PEG Data?

Most stock analysis websites and brokerage platforms provide PEG data, such as Yahoo Finance, Morningstar, Charles Schwab, etc. You can find it under the "Valuation" or "Financial Metrics" section of a stock.

But note: different platforms may use different growth rates (some use the past 5 years, others use forecasts for the next 3–5 years), so the same stock's PEG may vary across platforms. It's a good idea to understand how the platform calculates it before making a judgment.

Also, some platforms default to Forward PEG, others to Trailing PEG, and some let you choose. If you see a sudden change in a stock's PEG, first check if the growth rate data has been updated. Calculating it yourself can also deepen your understanding: find the P/E and growth rate, then plug them into the formula.