PEG Ratio
The PEG ratio adjusts the price-to-earnings ratio for expected earnings growth, helping investors determine whether a high-P/E stock is truly overvalued or simply priced to reflect superior growth prospects.
The PEG ratio was popularized by Peter Lynch, the legendary Fidelity Magellan fund manager who compounded returns at 29% annually from 1977 to 1990. Lynch observed that the P/E ratio alone could be misleading: a company growing earnings at 30% per year deserves a higher P/E than one growing at 5%, but the P/E ratio by itself does not account for this difference. Dividing P/E by the earnings growth rate (expressed as a whole number) normalizes for growth.
Lynch's rule of thumb was that a PEG ratio at or below 1.0 represents reasonable or attractive value — you are paying a P/E equal to or less than the growth rate. A PEG above 1.0 means you are paying a premium to the growth rate, which may be justified by other quality factors or may signal overvaluation. A PEG below 0.5 is a potential 'double discount' — strong growth at a low earnings multiple.
Applied to the S&P 500, the PEG framework helps explain why a company like UnitedHealth Group might trade at a P/E of 20 while growing earnings at 12-14% (PEG roughly 1.4-1.7) while Nvidia might trade at a P/E of 40 while growing earnings at 50%+ (PEG below 1.0 in high-growth years). In the latter case, the seemingly expensive P/E is actually cheap on a PEG basis if the growth rate is credible and sustainable.
The PEG ratio has important limitations. It depends entirely on the accuracy of growth rate forecasts, which are highly uncertain especially for cyclical or early-stage companies. Using 5-year projected EPS growth from Wall Street consensus — a common approach — embeds the collective optimism of sell-side analysts who have strong incentives to be bullish. Growth rates also mean-revert over time; a company growing at 40% today almost certainly will not sustain that rate for a decade.
Some analysts use a dividend-adjusted PEG, also called the 'PEGY ratio,' which adds the dividend yield to the earnings growth rate in the denominator. This modification recognizes that dividend income is a real component of total return and should factor into the valuation equation. For mature, lower-growth companies that pay substantial dividends, PEGY provides a more complete picture than raw PEG.