A stock with a P/E of 40 looks expensive next to one trading at 15 — until you learn the first is growing earnings 35% a year and the second is shrinking. The PEG ratio exists to settle exactly this argument. It divides the P/E by the earnings growth rate, giving you a single number that tells you whether you're paying a fair price for the growth you're buying. For screening growth stocks, it's one of the most underused filters available.
What the PEG Ratio Actually Measures
PEG is calculated as P/E divided by the annual EPS growth rate (expressed as a whole number). A stock with a P/E of 30 growing earnings at 30% has a PEG of 1.0. The rule of thumb popularized by Peter Lynch is simple: a PEG of 1.0 means the stock is fairly valued relative to its growth, below 1.0 suggests it may be undervalued, and above 2.0 suggests you're overpaying. The power of PEG is that it normalizes valuation across companies growing at wildly different rates. A 45 P/E is reasonable for a company compounding at 40%, but absurd for one growing at 8%. PEG captures that distinction in a way raw P/E never can, which is why growth investors lean on it to avoid both overhyped names and genuinely cheap compounders hiding behind high headline multiples.
Setting PEG Thresholds That Work
For a focused growth screen, set PEG between 0.5 and 1.5 as your core band. Below 0.5 is rare and usually signals either a data error or a market that doubts the growth will hold. The 1.0-to-1.5 range catches quality growers the market hasn't fully repriced. Pair PEG with an absolute growth floor — require forward EPS growth above 15% — so you don't catch low-multiple, low-growth stocks that technically pass on PEG but offer no real momentum. A company with a P/E of 12 and 10% growth has a PEG of 1.2, but it's not a growth stock. Adding the growth filter keeps your results genuinely growth-oriented. On DeltaScreener, combine a PEG ceiling of 1.5 with EPS growth above 15% and revenue growth above 10% to surface names that are both expanding and reasonably priced.
The Forward vs. Trailing PEG Trap
PEG is only as good as the growth number you feed it. Trailing PEG uses the last 12 months of earnings growth — a backward-looking figure that can be inflated by one-time gains, easy comparisons, or a recovery year that won't repeat. Forward PEG uses analyst estimates of future growth, which is more relevant but vulnerable to overly optimistic projections. The smart approach is to look at both. If a stock screens cheap on trailing PEG but expensive on forward PEG, the market is signaling that growth is decelerating. The reverse — cheap on forward, expensive on trailing — can flag a genuine inflection. Never trust a PEG below 1.0 without checking whether the growth denominator is sustainable. A 50% growth rate driven by a single blockbuster product launch will normalize, and your PEG will balloon overnight.
Where PEG Breaks Down
PEG is useless for companies with no earnings, since you can't compute a meaningful P/E — pre-profit growth names need price-to-sales instead. It also distorts for cyclical businesses, where peak-cycle earnings produce artificially low P/Es and misleadingly attractive PEGs right before a downturn. Financials and REITs require adjusted metrics rather than standard PEG. And the ratio says nothing about balance sheet risk: a company can have a perfect PEG of 0.9 while carrying dangerous leverage. Treat PEG as a valuation-versus-growth filter, not a complete thesis. Always layer it with a debt-to-equity check, a free cash flow screen, and a look at margin trends. The cleanest PEG in your results means nothing if the company is burning cash to fund the growth that's making the ratio look good.
Building a PEG-Based Growth Screen
A practical starting screen looks like this: PEG between 0.5 and 1.5, forward EPS growth above 15%, revenue growth above 10%, debt-to-equity below 1.0, and positive free cash flow. This combination filters out value traps, pre-profit cash burners, and over-leveraged names in one pass, leaving you with profitable growers trading at sane valuations. From there, sort by PEG ascending and review the lowest figures manually — the cheapest names relative to growth deserve the closest scrutiny because that's where both the best opportunities and the most dangerous illusions live. Run this screen on DeltaScreener at deltascreener.com/screener, save it as a recurring filter, and rerun it after each earnings season to catch growth stocks the market reprices before the crowd notices.