The P/E ratio has a blind spot: it makes fast-growing companies look "expensive" and slow ones look "cheap." The PEG ratio fixes that. It's the single best tool for judging whether a growth stock's high price tag is actually justified - and it's simple enough to work out in your head.
What is the PEG ratio?
The PEG (price/earnings-to-growth) ratio takes the P/E ratio and divides it by the company's expected earnings growth rate:
PEG = P/E ÷ annual EPS growth (%)Example: a stock with a P/E of 30 growing earnings 30% a year has a PEG of 1.0 (30 ÷ 30). Another with the same P/E of 30 but growing just 10% has a PEG of 3.0 - identical P/E, but three times more expensive for the growth you actually get. If you're rusty on the first half of that equation, start with what is a good P/E ratio.
How to read the PEG ratio
| PEG | Interpretation |
|---|---|
| Under 1 | Potentially undervalued - you're paying little for the growth |
| Around 1 | Fairly valued - price roughly matches growth |
| Over 1 | Growth is already priced in |
| Over 2 | Expensive unless growth accelerates |
Legendary investor Peter Lynch popularized the rule of thumb that a fairly priced growth company should trade around a PEG of 1.0.
Why PEG beats P/E alone
Judged on P/E alone, a stock like Nvidia (NVDA) or Microsoft (MSFT) can look wildly "overpriced" at 35-50x earnings. But if profits are compounding 30-40% a year, that high P/E may be perfectly reasonable - the PEG reveals it. Meanwhile a "cheap-looking" stock like Coca-Cola (KO) at a P/E of 24 while growing 5% has a PEG near 5, which is anything but cheap. PEG is what separates an expensive stock from an overpriced one.
A worked example
| Stock A | Stock B | |
|---|---|---|
| P/E | 15 | 40 |
| EPS growth | 5% | 40% |
| PEG | 3.0 | 1.0 |
Stock A looks cheaper on P/E, but Stock B is the better value for its growth. This is exactly how the market can pay 40x earnings for Amazon (AMZN) or Apple (AAPL) without it being "irrational."
The limitations (read before you trust it)
PEG is powerful but not magic:
- It relies on growth estimates - garbage in, garbage out. Analyst forecasts are often wrong.
- It's useless for no-growth, cyclical, or unprofitable companies (no meaningful growth rate to divide by).
- It's sensitive to which growth number you use - next year's estimate vs a 5-year rate can swing the PEG wildly.
- It ignores debt, dividends, and risk. A low PEG on a debt-laden company isn't a free lunch.
Treat PEG as one lens, not a verdict.
How to use PEG (with P/E and quality)
1. Start with P/E to see the raw price tag.
2. Divide by growth to get PEG - is that price justified?
3. Sanity-check the growth estimate - is it realistic, or a hopeful forecast?
4. Confirm quality - margins, debt, and a real competitive edge.
Screen for reasonable P/E and strong growth in the free stock screener, then apply PEG to your shortlist. For the full checklist, see how to find undervalued stocks.
FAQ
What is a good PEG ratio? Around 1.0 or below is generally considered attractive - you're paying a fair price (or less) for the company's growth. Is a lower PEG always better? Usually, but not blindly. A very low PEG can rest on an over-optimistic growth estimate that won't hold up. What's the difference between P/E and PEG? P/E measures price against current earnings; PEG adjusts that for how fast earnings are growing. PEG is the fairer yardstick for growth stocks. Can the PEG ratio be negative? Yes - if earnings are shrinking (negative growth), PEG turns negative and becomes meaningless. Skip it for companies with falling profits.Final Take
The PEG ratio is the antidote to P/E's biggest flaw: it lets you pay up for growth without overpaying. A high P/E isn't automatically expensive, and a low P/E isn't automatically cheap - PEG is what tells the difference. Aim for around 1.0 or less, always question the growth estimate behind it, and pair it with a quality check. Put it to work now in the stock screener.
This article is for informational purposes only and is not financial advice. Always do your own research before investing.
