PEG Ratio: What It Tells You About Growth and Price

What is the PEG ratio?

The PEG ratio adjusts a stock's price-to-earnings ratio for how fast the company is growing, to answer a question the P/E cannot: is a high multiple justified? It divides the P/E by the earnings growth rate, so a company with a P/E of 30 growing at 30 percent a year has a PEG of 1.0, while the same P/E on a company growing at 10 percent gives a PEG of 3.0. The idea is that growth earns a higher price.

The P/E ratio on its own has a blind spot. It tells you how expensive a stock is relative to current earnings, but says nothing about the future. A P/E of 30 looks costly next to a market average near 18, yet it can be perfectly reasonable if the company's earnings are doubling every few years. The PEG ratio tries to close that gap by putting price and growth in the same number.

The measure is associated with Peter Lynch, who liked to compare a company's P/E directly with its growth rate as a rough sanity check. The intuition is appealing: a business growing earnings at 20 percent a year arguably deserves to trade around 20 times earnings, giving a PEG near 1.0. It is a quick way to ask whether you are paying a fair price for the growth you expect, and it connects naturally to the tension between growth and value investing.

How is the PEG ratio calculated?

The PEG ratio is the price-to-earnings ratio divided by the annual earnings growth rate, with the growth rate written as a plain number rather than a percentage. So a growth rate of 20 percent is entered as 20, not 0.20.

PEG ratio = P/E ratio / Annual earnings growth rate

Say a stock has a price-to-earnings ratio of 24 and analysts expect its earnings to grow 20 percent a year. Divide 24 by 20 and you get a PEG of 1.2. The stock trades at a slight premium to its growth rate, which many investors would read as close to fair, perhaps a touch expensive.

Reading down the table shows the point of the ratio. Company C has the lowest P/E, at 18, and looks cheapest on that measure alone, but its slow growth gives it the highest PEG, at 3.0, so it is actually the most expensive relative to how fast it grows. Company A, with the scariest P/E of 30, has the most attractive PEG. The ratio reorders which stock looks dear once growth enters the picture.

What counts as a good PEG ratio?

A PEG ratio near 1.0 is the traditional marker of fair value, with below 1.0 suggesting a stock is cheap relative to its growth and above 2.0 suggesting it is expensive. These thresholds are rough rules of thumb, useful for a first pass, but they carry far less authority than their neat numbers imply.

The logic behind a PEG of 1.0 is that a company's earnings multiple should roughly match its growth rate. Pay 20 times earnings for 20 percent growth, and the price and the growth are in balance. Pay 40 times earnings for 20 percent growth, a PEG of 2.0, and you are paying up for growth that may not justify the premium. Pay 10 times for 20 percent growth, a PEG of 0.5, and you may have found a bargain, if the growth is real.

The word "rough" is doing real work in every one of those rows. The PEG is a screening tool and a conversation starter, not a valuation on its own, because it compresses a great deal of uncertainty into one tidy figure. A PEG of 1.0 built on a shaky forecast is worth far less than a PEG of 1.5 built on a reliable one. That fragility is the whole problem, and it deserves its own section.

The trap: garbage growth in, garbage PEG out

The main trap in the PEG ratio is that it depends entirely on a growth estimate, and growth estimates are frequently wrong, so a bad input produces a confidently wrong output. The P/E half of the ratio is a hard fact you can look up. The growth half is a forecast, often reaching years into the future, and it is the part that swings the answer most.

Watch how sensitive the ratio is. Take a stock with a P/E of 30. If it grows at 30 percent a year, its PEG is a fair-looking 1.0. But growth forecasts are estimates, and if the true rate turns out to be 20 percent, the PEG was really 1.5, and at 15 percent it was 2.0, a stock that looked fairly priced but was actually expensive. Nothing about the price changed; only the assumed growth did, and the verdict flipped.

Same stock, same price, three different conclusions, driven entirely by a forecast no one can be sure of. This is why the PEG carries a false precision. Reporting a PEG of 1.2 to one decimal place implies a level of accuracy the inputs cannot possibly support, since the growth rate underneath might be off by a third. The neat number lends a borrowed authority to what is really a guess.

The other weakness is that there is no standard growth rate to use. One analyst might use next year's expected growth, another a five-year estimate, another the past growth rate, and each produces a different PEG for the identical stock. The defense is to treat the PEG as a rough filter, always ask where the growth number came from and how firm it is, and never let one decimal place substitute for judgment about whether the growth is achievable. Reading it beside market expectations and a hard look at the business is what keeps the ratio honest.

Where to go from here

The PEG ratio is a useful way to put price and growth in a single view, but it is only as good as the growth forecast underneath, which is often the weakest link. Start with the price-to-earnings ratio to understand the P half, then read market expectations to think clearly about the growth the price is assuming. When you are ready, use the Tenet stock screener to compare growth and valuation together, and always question the estimate before trusting the ratio.