What is the PEG Ratio
The PEG ratio is a refinement of the classic price-to-earnings ratio. It takes the PE multiple and divides it by the expected growth in earnings, producing a figure that attempts to show how much an investor pays for each unit of future growth.
A company trading at a PE of 20 with expected earnings growth of 20 percent carries a PEG of 1.0, often read as fairly priced against its growth. A PE of 20 with growth of 10 percent gives a PEG of 2.0, implying the market is paying generously for slower growth.
The ratio gained fame as a quick screen for growth at a reasonable price, comparing stocks that trade at very different multiples on an equal growth-adjusted footing.
This guide sets out the definition of the PEG ratio, shows the formula and its variants, walks through worked examples, explains when it helps, and flags where the ratio can mislead.
The enduring appeal of the PEG is that it frames the price you pay against the pace the company is expected to grow. A high multiple stops looking outrageous once the growth is high too, and a low multiple stops looking like a bargain once the growth stalls. Bringing price and pace together on one scale is what the ratio does in a single number, and it is why the tool earned its place in the growth-at-a-reasonable-price school of stocks.
Definition
The PEG ratio is the price-to-earnings ratio divided by the expected percentage growth in earnings per share over a coming period, usually one to five years.
It attaches growth to the multiple. Whereas the PE ratio asks how many times earnings you pay, the PEG asks how much you pay for each point of growth that the market expects.
A reading around one suggests the market has priced the stock roughly in line with its growth; below one often flags a cheap price, and well above one a rich one, always subject to the quality of the growth forecast itself.
Because it is the ratio of two ratios, the PEG is scale-free: a firm trading at twenty times earnings growing at twenty percent, and one at ten times earnings growing at ten percent, both show a PEG of one. Equality, and the inequality that surfaces when the two diverge, is precisely what the metric is engineered to reveal.
Formula
PEG Ratio = PE Ratio / Expected Earnings Growth (%) For example: - PE Ratio = 15, expected growth = 10% -> PEG = 1.5 - PE Ratio = 15, expected growth = 20% -> PEG = 0.75 The growth figure is an expectation, usually the consensus forecast of analysts for earnings per share over the next one or two years, and its definition drives the result. Variants use the trailing growth, the forward growth, or the five-year expected growth, so the same stock can report different PEG values depending on which growth input was chosen. A common shorthand puts the emphasis on the quarter of a unit: a reading near one is cost-proportional to growth, above two is routinely expensive, and a PEG that falls well below one may be compensating the growth with unseen risk. These anchors are rough, so read them as context, not as verdicts.
Example
Consider two software firms in the same market. Nova trades at a PE of 30 with expected earnings growth of 30 percent; Cedar trades at a PE of 15 with expected growth of 8 percent.
Nova's PEG is 30 divided by 30, or 1.0. Cedar's PEG is 15 divided by 8, or 1.875. Despite looking far cheaper on the PE line, Cedar is more expensive per unit of growth because its growth is expected to be so much slower.
If a third firm, Pine, trades at a PE of 20 with 25 percent expected growth, its PEG of 0.8 sits below one, signalling the market may be pricing its expansion cheaply.
These numbers make the point: the PEG brings the multiples onto a common scale, but the comparison is only as trustworthy as the forecasts feeding the denominator.
Imagine Nova's growth forecast is later cut from 30 to 15 percent. Its PEG climbs from 1.0 to 2.0 without any change in the share price, which is the real fragility of the tool: the whole message rests on a number the market is forever revising.
How to Use It
Use the PEG as a filter rather than a verdict. Screen candidate stocks for those with a PEG under one or near one, then check the earnings history and the strength of the growth story before believing the screen.
Prefer it for growing businesses where the PE looks high in absolute terms. For a mature firm growing in single digits, the PEG loses its usefulness, since the divisor becomes small and volatile.
Compare PEG values inside an industry and demand the same growth source for every stock, whether trailing, forward, or five-year, so the ratio stays consistent.
Also weigh the sustainability of the growth, not just its size. A firm that has beaten estimates quarter after quarter is a more believable growth story than one inheriting a bold target with no track record, so combine the ratio with the recent earnings history it is largely extrapolating.
Limitations
The PEG is only as good as its growth number, and growth forecasts are frequently optimistic. A single analyst revision can swing the ratio from 0.9 to 1.4 overnight with no change in the price of the shares, which is why the reading should be treated as one straw in the wind rather than a fixed fact.
When growth is very high or negative, the ratio behaves badly. A firm growing 80 percent yields a tiny PEG that flatters its risk, and a firm with flat or falling earnings gives no meaningful number at all.
The ratio also ignores the quality of growth, the cash generated, the debt load, and the competitive moat, so a cheap PEG must never replace the hard questions.
In practice the PEG looks at only the price and the growth grade, and nothing else. A generous reading can hide a heavy balance sheet or a shrinking cash flow, and a stern one can bury a genuine compounder, so the single figure should start the analysis rather than end it. Pair the PEG with the source and confidence of the growth estimate, the strength of the balance sheet, and the durability of the cash behind the candidate before treating any low reading as a verdict.
Key Takeaways
- The PEG divides the PE multiple by expected earnings growth per share.
- Around one is the classic fair value against its own growth.
- The ratio inherits every weakness of the forecast that feeds it.
- Below one flags a candidate for a deeper look, not a buy signal.
- Negative or extreme growth makes the PEG meaningless.
- Confirm the growth source matches across every stock before you weigh the figures.
What is the PEG Ratio FAQ
What is a good PEG ratio?
A reading near one or below is commonly treated as fairly or cheaply priced relative to growth, while above 1.5 is considered rich. The threshold is a guide, not a rule, and should be confirmed against the industry and the forecast quality.
How is the PEG different from the PE?
The PE alone tells how many years of current earnings the price implies, while the PEG folds the expected growth in. A high PE can be justified when growth is high, and a low PE can be a trap when growth is slowing.
Which growth should I use in the PEG?
Be consistent. Forward one-year, forward two-year, and five-year growth all appear in sources, and each changes the number materially. Pick one definition and hold it across the stocks you compare, so any difference in the PEG reflects the prices and not the growth convention.
Does a low PEG guarantee a good buy?
No. It only says the price is modest against the expected growth. If the forecast is wrong, or the growth is poor quality or highly risky, the low PEG is misleading, so it is a screen, never a conclusion.
Can the PEG be negative?
When earnings growth is negative the divisor turns negative and the ratio loses its meaning. In that case fall back on the PE and the cash flow, which still work when growth is not.
Is the PEG useful for mature firms?
Weakly. With single-digit growth the divisor is small and the ratio unstable, so the PEG shines brightest for growing companies and dulls for steady, low-growth businesses.
Where does the growth number come from?
Usually the consensus of the analysts covering the stock, published by data vendors. Check the date and the sample, because a few bold forecasts move the average and distort the PEG, so trace the number back to its source.
Should the PEG use trailing or forward growth?
Forward looks ahead and is the common choice, but it leans on estimates, while trailing uses the past and is more stable. Choose deliberately, note the choice, and never mix the two styles across the names you compare.