What the PEG Ratio Measures
A P/E of 20 tells you how many dollars you pay per dollar of current earnings, but it says nothing about how fast those earnings are growing. A company compounding EPS at 25% a year can deserve a 20x multiple; one growing at 5% usually cannot. The PEG ratio fixes that blind spot by dividing the P/E by the expected annual growth rate, so a 20x multiple on 15% growth lands at 20 ÷ 15 = 1.33.
Peter Lynch popularized the rule behind the number: a stock is roughly fairly priced when its P/E equals its growth rate, which is another way of saying PEG = 1. Below 1, you are paying less than a dollar per point of growth. Above 1.5, you are paying ahead of the growth on offer, and the price already embeds several years of execution.
The tool above computes the ratio from four inputs and adds the context a bare number lacks: the underlying P/E, the fair price implied by a PEG of exactly 1, projected EPS over your horizon, and the return you would earn if the multiple holds while earnings compound. For a deeper look at the earnings figure itself, the EPS calculator breaks down basic and diluted EPS from net income and share counts.
The Math Behind the Tool
The core formula is short: PEG = (Price ÷ EPS) ÷ growth rate in percent. With the default inputs — a $50 share price, $2.50 of EPS, and 15% expected growth — the P/E is 20.0x and the PEG is 20 ÷ 15 = 1.33. The figure reads as dollars of multiple paid per point of annual growth, which makes very different stocks directly comparable.
The fair-price line comes from the same algebra. Setting PEG to 1 means the fair P/E equals the growth rate, so fair price = EPS × growth. At the defaults that is $2.50 × 15 = $37.50, meaning the market price sits 33.3% above a PEG-1 valuation — or, seen from the quote, the fair price is 25% below where the stock trades. Small changes in the growth estimate move that fair price one-for-one.
The projection half of the output compounds EPS at your growth rate for the horizon you enter, then re-applies the current multiple: $2.50 at 15% for five years becomes $5.03, and at a constant 20x the implied price is $100.57. That works out to a 15.0% annual return — exactly the growth rate, because the multiple never moved. The compound growth calculator runs the same exponential math on any metric you feed it.
Reading PEG Verdicts
The bands the tool uses follow the conventions most value investors apply. Below 0.5 sits deep-value territory, where the market loudly doubts the growth estimate — often for a reason. Between 0.5 and 1.0 the stock counts as undervalued relative to growth. The 1.0–1.5 range reads as fairly valued, 1.5–2.5 as richly valued, and anything above 2.5 means the multiple prices in growth that has not shown up in results yet.
Holding a 20x P/E fixed shows how fast the verdict moves with the growth input: 10% growth scores a PEG of 2.00, 15% lands at 1.33, 20% reaches 1.00, 25% prints 0.80, and 30% gets down to 0.67. Ten points of growth can swing the ratio by two-thirds, which is why serious PEG work spends most of its effort on the growth estimate rather than on the market price.
Lynch's own shortcut — fair P/E equals the growth rate — gives the same answer from the other side. On $2.50 of EPS, fair prices run $20 at 8% growth, $30 at 12%, $37.50 at 15%, $50 at 20%, and $62.50 at 25%. If you want a floor grounded in balance-sheet assets rather than growth forecasts, the Graham number calculator prices the same stock from book value and earnings floors.
Trailing vs Forward PEG
Trailing PEG divides the P/E built on the last twelve months of reported EPS by a growth rate; forward PEG uses estimated EPS for the next twelve months and the growth expected beyond that. The two can disagree sharply around an earnings inflection. A retailer exiting a weak year may show a scary trailing PEG of 2.4 and a forward PEG of 1.1 if earnings are guided to rebound.
Forward numbers rely on forecasts, and forecasts inherit optimism. Analyst consensus tends to run high early in the fiscal year and gets revised down as reporting dates approach. A practical discipline is to compute both versions: if only the forward PEG looks cheap, the stock is a bet on execution, and the size of the gap between trailing and forward tells you how much has to go right.
For dividend payers, a small adjustment often changes the verdict. The dividend-adjusted PEG divides the P/E by growth plus dividend yield: a 20x multiple with 12% growth and a 3% yield scores 20 ÷ 15 = 1.33, versus 1.67 on the raw formula. That correction matters most for slow-growth, high-yield names where distributions are half the return. To value a stock primarily on its dividends, the dividend discount model calculator is the more direct tool.
Estimating the Growth Rate
The growth input does all the heavy lifting, so anchor it in evidence. Three defensible sources are a historical 3-to-5-year EPS CAGR, analyst consensus for the next three to five years, and the sustainable growth rate — return on equity multiplied by the retention ratio. When the three disagree widely, the historical figure usually deserves the most weight, because it is the only one already earned.
Compounding makes the horizon choice matter. At 15% growth, $2.50 of EPS becomes $3.80 in three years, $5.03 in five, and $10.11 in ten. A ten-year run at 15% is a rare feat — few listed companies sustain it — so long horizons deserve haircut growth rates. The EPS growth calculator computes the historical CAGR from two EPS readings, and the CAGR calculator generalizes the same math to any series.
Sanity-check any estimate against revenue growth. EPS growing 20% a year on 6% revenue growth means margins must expand every period, and margin expansion has limits. The cleanest PEG candidates show earnings growth within a few points of revenue growth, flat-to-rising margins, and a growth rate you can trace to unit demand or pricing rather than to one-off items like asset sales or tax timing.
Where PEG Breaks Down
Cyclical companies poison the ratio at the worst moment. At an earnings peak, the trailing P/E looks low and growth looks strong, so the PEG prints an attractive 0.8 — right before earnings collapse. The same stock at the trough shows a terrible PEG just before the recovery. For cyclicals, normalize EPS across a full cycle or drop the PEG entirely in favor of price-to-book or mid-cycle margins.
The ratio also fails on negative or near-zero earnings, where the P/E has no meaning and neither does anything divided by it — the tool returns an explicit undefined result instead of a number. Banks are a third weak case: their EPS growth is constrained by regulated capital, so a cheap PEG on a bank usually says nothing at all. Fair-value frameworks built on cash flows, like the intrinsic value calculator, handle these odd cases more gracefully.
High-octane growth deserves skepticism even when the arithmetic works. A 30x multiple on 30% growth scores a PEG of 1.0, and the projection says $3.00 of EPS becomes $11.14 in five years with an implied price near $334 — but only if nothing interrupts the run. One missed year, one margin slip, and the multiple compresses faster than earnings grow. Use PEG as a screen, then verify the durability of the growth itself.
PEG Inside a Full Valuation Stack
A single ratio is a starting point, never a verdict. The strongest case for a stock combines a reasonable PEG with a discounted cash flow or dividend model pointing the same direction, a return estimate above your hurdle rate, and a balance sheet that can survive a bad year. When two or three independent methods agree, the margin of safety is real; when only the PEG looks cheap, the market is probably pricing a risk you have not found yet.
A worked comparison shows the ratio doing its job. Stock A trades at $80 on $4.00 of EPS growing 8%: a 20x multiple, PEG 2.50, and an 8.0% implied return if the multiple holds. Stock B trades at $45 on $1.50 of EPS growing 20%: a 30x multiple, PEG 1.50, and a 20.0% implied return. The cheaper-looking stock on P/E is the worse deal, which is exactly the trap PEG exists to catch.
The return side needs a hurdle rate. The CAPM calculator turns beta and market expectations into the return you should demand, and the expected return calculator frames the probability-weighted alternative. If a PEG-implied 15% return sits barely above an 11% hurdle, the position barely earns its keep; if it clears the hurdle by six points, the price is doing real work for you.
Screening and Portfolio Practice
Screens built on PEG need guardrails to stay honest. A common filter asks for PEG below 1.0 with growth above 10% and positive free cash flow, which filters out value traps where a low PEG merely reflects stalled estimates. Re-running the screen after each earnings season matters more than the exact thresholds, because growth estimates decay faster than prices move.
Model multiple compression before you trust an implied return. The default projection assumes the 20x multiple survives five years, producing 15.0% a year. If the market re-rates the stock down to 15x over that period, the implied price falls to $75.43 and the return drops to 8.6%; if sentiment improves to 25x, the price reaches $125.71 and the return rises to 20.2%. The earnings did identical work in all three cases — the multiple decided the outcome.
Size positions by conviction and cross-check scale. A cheap PEG on a $400 million micro-cap carries liquidity and single-customer risks a large cap does not, so the market capitalization calculator belongs in the same workflow. Rebuild the ratio every quarter with fresh EPS, keep trailing and forward versions side by side, and write down the growth assumption you used — the fastest way to learn which estimates you can trust is to reread your own.