The Ratio That Fixes What P/E Gets Wrong
Two stocks. One trades at a P/E of 15, the other at 30. Obvious which is cheaper, right? Now add one fact: the first grows earnings 5% a year and the second grows 30%. Suddenly the expensive-looking stock is the better deal. That is the exact problem the PEG ratio was built to solve. P/E alone tells you what you pay for a dollar of current earnings. It says nothing about how fast those earnings are growing.
The PEG ratio divides the price-to-earnings ratio by the company's earnings growth rate. The formula is simple: PEG equals P/E divided by the annual EPS growth rate (entered as a whole number). Take the second stock above: a P/E of 30 divided by a 30% growth rate gives a PEG of 1.0. The first stock: a P/E of 15 divided by 5% growth gives a PEG of 3.0. By this measure the high-P/E growth stock is three times cheaper than the low-P/E slow grower.
The widely cited benchmark, popularized by legendary fund manager Peter Lynch, is that a PEG around 1.0 suggests a stock is fairly priced relative to its growth. Below 1.0 hints at undervaluation. Above 1.0, and especially above 2.0, suggests you are paying a premium that growth may not justify.
Here is a clean comparison. A company with a P/E of 24 and 12% growth has a PEG of 2.0, looking expensive for its growth. A company with a P/E of 18 and 20% growth has a PEG of 0.9, looking like a relative bargain even though its raw P/E is also elevated. The PEG ratio levels the playing field between a steady utility and a fast-expanding tech name, letting you compare valuations across very different growth profiles with a single number.
The intuition behind the benchmark is elegant. A PEG of 1.0 implies the market is charging you exactly one unit of P/E for each percentage point of growth, a roughly fair exchange. Pay more than that and you are betting growth will exceed expectations. Pay less and the market is, in effect, handing you growth at a discount. This is why a seasoned investor can glance at a stock trading at 40 times earnings and not flinch, provided it is compounding earnings at 40% a year, while treating a 12 times P/E utility growing at 2% as the genuinely expensive option on a growth-adjusted basis.
Used well, PEG keeps you from dismissing great growth companies as too expensive and from overpaying for slow ones that merely look cheap. It is the antidote to the lazy instinct that low P/E always means cheap and high P/E always means expensive, an instinct that has cost value-minded investors a fortune in missed compounders over the years.