Selurio · Blog
What the PEG Ratio Is — and How Peter Lynch Made It Popular
The classic P/E metric (price-to-earnings) tells you how much you pay for every euro of a company's earnings. But it has an obvious gap: a company growing 30% a year deserves a higher P/E than one standing still. P/E on its own does not distinguish the two.
The PEG ratio solves exactly this problem — and it is perhaps the best-known tool popularized by the legendary fund manager Peter Lynch in his book One Up on Wall Street.
The formula
PEG = (P/E) / Annual earnings growth (%)
The idea is elegant: you divide the valuation (P/E) by the growth that justifies it. If a company with a P/E of 20 grows 20% a year, its PEG is 1.0 — you pay exactly what its growth "is worth."
A few examples:
- P/E 15, growth 15% → PEG = 1.0 (fair)
- P/E 20, growth 15% → PEG = 1.33 (slightly above fair value)
- P/E 10, growth 20% → PEG = 0.5 (cheap relative to growth)
Run your own numbers in the PEG calculator.
Peter Lynch's framework
Lynch offers a simple interpretation — with an explicit legend, so there is no ambiguity:
| PEG zone | Interpretation per Lynch |
|---|---|
| PEG < 1 | Potentially undervalued relative to growth |
| PEG ≈ 1 | Fair value |
| PEG > 1 | Potentially overvalued relative to growth |
Lynch's well-known maxim is that the price of a growing company is reasonable when its P/E roughly equals its growth rate — that is, when the PEG is around 1.
The traps nobody warns you about
PEG is useful, but it is only as good as the numbers you feed it. Here is where it goes wrong:
- Growth is a forecast, not a fact. The denominator is expected growth. If you get that wrong — and the future is hard to predict — the whole PEG is meaningless. "Garbage in, garbage out."
- Negative or zero growth. For a company in decline the formula breaks (division by zero or a negative number). PEG is simply not applicable to such companies.
- Very high P/E. With a P/E above 50, a small change in growth moves the PEG dramatically — reliability drops.
- Different businesses. PEG works best for stable, growing companies. For cyclical, financial, or deeply undervalued businesses the picture is more complex.
That is why the Selurio calculator shows the zones with an explicit color legend and warns at extreme values — so that PEG is not a black box that spits out a "verdict."
PEG is not a signal
An important note in the spirit of Selurio: PEG is not a "buy" button. It is one lens among many — a way to put valuation in the context of growth. Serious analysis also looks at the balance sheet, cash flow, the competition, and the quality of management. Neither we nor the calculator give recommendations to buy or sell any particular stock.
Continue reading
- Compound interest — the eighth wonder of the world — why growth compounded over time is so powerful.
- Stocks vs. ETFs vs. crypto — where PEG analysis makes sense and where it does not.
- All of Selurio's calculators — the tools for your analysis.
Subscribe
Like the depth? Subscribe to the newsletter — new calculators and educational articles, no signals and no spam.