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Peter Lynch's PEG Rule: Why Under 1.0 Means Buy
Peter Lynch ran Fidelity Magellan from 1977 to 1990 and compounded the fund at 29.2% a year. He kept one valuation shortcut close at hand: if a stock's PEG ratio falls below 1.0, it deserves a closer look.
The PEG heatmap ranks every S&P 500 stock by PEG in a single view, so Lynch's screen takes seconds rather than a spreadsheet.
How Lynch Built His Record
In thirteen years at Magellan, Lynch grew assets from $18 million to $14 billion and beat the S&P 500 in eleven of those years. The fund ended as the largest in the world. An investor who put $10,000 in on day one held roughly $280,000 when Lynch retired, aged 46, to spend time with his daughters.
His method was peculiar for a Wall Street manager. He noticed products his wife Carolyn brought home, then did the balance-sheet work. L'Eggs pantyhose (through Hanes) became a multi-bagger that way. Taco Bell, Dunkin' Donuts, and Fannie Mae came from the same habit. Chrysler, his largest position in the early 1980s, came in as a turnaround around $2 a share and sold in the mid-$40s.
At retirement the fund held more than 1,400 stocks, a portfolio so broad that Lynch later joked he was running an index fund with alpha skimmed on top. He left one sentence that captures the discipline: "Behind every stock is a company. Find out what it's doing." The PEG rule he popularised is the numerical version of that instruction, a quick read on whether a company's price matches the rate at which its business is actually growing.
The Rule: PEG Below 1.0
Lynch's original formulation appears in chapter 10 of One Up on Wall Street:
The P/E ratio of any company that's fairly priced will equal its growth rate. If the P/E ratio is less than the growth rate, you may have found yourself a bargain.
Translate that into math and you get the PEG ratio:
PEG = P/E ÷ Earnings Growth Rate
A stock with a P/E of 20 growing earnings at 25% a year has a PEG of 0.80. Lynch called that attractive. A stock at P/E 30 growing 10% has a PEG of 3.0, and he called that very negative. Half the growth rate is a strong signal. Twice the growth rate is a warning.
The rule scales because it frees you from debating whether a high P/E is justified: the growth rate decides. A utility at P/E 12 growing 3% has a PEG of 4.0, which Lynch read as expensive. A software firm at P/E 35 growing 40% has a PEG of 0.88, a candidate worth opening the 10-K on.
The Dividend-Adjusted "Lynch Ratio"
Beating the Street introduced a variation:
Find the long-term growth rate, add the dividend yield, divide by the P/E ratio. Less than 1 is poor, 1.5 is okay, but what you're really looking for is 2 or better.
This inverts the classic PEG. Take a P/E of 15 on a stock growing 12% and paying a 3% dividend: (12 + 3) / 15 = 1.0, which Lynch called poor. He wanted (growth + yield) / P/E to sit at 2.0 or higher. A stock at P/E 10 growing 15% with a 5% dividend clears that bar: (15 + 5) / 10 = 2.0.
The dividend adjustment matters because a mature stalwart throwing off 4% in cash looks different from a growth name paying nothing. Lynch's ratio rewards the cash return; the classic PEG ignores it entirely.
Screeners almost never report this version. Yahoo Finance and most brokers show the classic PEG. Running Lynch's actual rule means grabbing the dividend yield separately and doing the arithmetic. The gap between what he wrote and what your broker shows is bigger than most people realise.
The Six Categories Behind the Rule
Lynch filed every stock into one of six buckets. The PEG test means different things in each.
| Category | Typical growth | PEG relevance |
|---|---|---|
| Slow growers | 2-4% | Owned for yield, not for PEG |
| Stalwarts | 10-12% | PEG works; trim when it tops 2.0 |
| Fast growers | 20-25%+ | PEG below 1.0 is the target |
| Cyclicals | Volatile | PEG misleads at the peak of the cycle |
| Turnarounds | Reset from low | PEG breaks; earnings are unstable |
| Asset plays | Irrelevant | Value sits off the income statement |
The rule was built for fast growers and stalwarts. That was Magellan's sweet spot and where the Chrysler, Hanes, and Philip Morris positions lived. Applied to a cyclical at peak earnings, the rule flatters the stock a moment before earnings collapse.
Where the Rule Stops Working
Three scenarios break it.
Unprofitable companies. PEG divides P/E by earnings growth. If earnings hover near zero, the P/E explodes. If earnings are negative, PEG turns negative and loses meaning. Lynch's examples were all profitable businesses. Modern screeners return garbage PEG values for pre-profit SaaS and biotech, and readers quote them anyway.
Cyclicals at the top. Earnings inflate at the peak, the P/E collapses, and PEG looks cheap. Then the cycle turns. Lynch warned to treat low cyclical P/Es as inverted signals, not bargains.
Mismatched growth estimates. PEG depends on the growth input, and analyst earnings forecasts miss regularly. Yahoo, Morningstar, and Zacks use different growth windows, so "PEG under 1.0" on one screener can read above 1.5 on another. Academic work (Estrada 2004; Schatzberg and Vora 2008) found most of the low-PEG premium is already absorbed by the value and small-cap factors it overlaps with, not by a standalone PEG edge.
One more Lynch admission matters. In a 1995 Barron's interview he conceded investors in Magellan earned far less than 29% because they traded in and out. The tool was sound. The behaviour was not, and no ratio in the world fixes that.
Start with the PEG heatmap to see which S&P 500 names sit below Lynch's 1.0 line today, then open the underlying 10-K on any that clear it.
Frequently Asked Questions
Did Peter Lynch invent the PEG ratio? No. Mario Farina used a version of the ratio in a 1969 book, "A Beginner's Guide to Successful Investing." Lynch popularised it in "One Up on Wall Street" in 1989 and turned it into a rule with a specific threshold at 1.0.
Does Lynch's PEG rule work on tech stocks? It works on profitable tech stocks with stable growth. It breaks on pre-profit software and biotech because the earnings denominator is near zero or negative, and tiny changes in the growth estimate swing the PEG wildly. Lynch wrote for businesses with real earnings.
What is the Lynch ratio and how is it different from PEG? The Lynch ratio, from "Beating the Street," is (earnings growth plus dividend yield) divided by P/E. The classic PEG is the inverse: P/E divided by growth. Lynch's version aims for 2.0 or higher; the classic PEG aims for under 1.0. Both signal the same idea from opposite ends.
Is a PEG below 1.0 always a buy signal? No. The rule fails on cyclicals at the earnings peak, on companies where growth estimates are unreliable, and on businesses with poor balance sheets. Lynch paired PEG with checks on debt, insider buying, and the underlying story. Read PEG as a starting point, not a verdict.