"Is 20x P/E expensive?" is one of those interview questions that sounds simple and isn't. The honest answer is always the same: it depends — and knowing exactly what it depends on is what separates a candidate who sounds like they've memorized a definition from one who actually understands valuation. A price-to-earnings ratio, on its own, is just a ratio of two numbers. Whether 20x is high or low only becomes meaningful once you compare it against something.
Why a P/E Number Alone Doesn't Tell You Anything
Two companies can trade at an identical 20.0x P/E and be, in economic terms, complete opposites. One might be a fast-growing software company where investors are paying 20 times this year's earnings because they expect those earnings to compound for a decade. The other might be a slow-growing utility where 20 times earnings looks rich because growth is barely keeping pace with inflation. The number is the same; what it implies about the business is not. This is the core idea behind Case 34: "Is 20x P/E Expensive?", which walks through exactly this comparison using two companies at the same multiple but very different growth rates and industries.
The Growth Adjustment: PEG Ratio
The first and most common adjustment analysts make is to divide the P/E ratio by the company's expected earnings growth rate, producing the PEG ratio (Price/Earnings-to-Growth). A PEG near 1.0x is often read as roughly "fair" — the multiple is broadly in line with the growth being priced in. A PEG well below 1.0x can suggest the market is underpaying for growth, while a PEG well above 1.0x suggests investors are paying a steep premium for very little of it. It's a rough heuristic, not a precise valuation tool, but it's usually the fastest way to reframe a raw multiple into something comparable across companies with different growth profiles.
The Industry Adjustment: Relative P/E
Growth isn't the only thing that varies by company — entire industries carry structurally different multiples because of differences in capital intensity, margin stability, and growth ceilings. Software and healthcare businesses have historically traded at higher P/E multiples than utilities or industrials, largely because their earnings are less capital-intensive and their growth runways are longer. That's why analysts also compute a relative P/E — the company's multiple divided by its industry average — before drawing any conclusions. A 20x P/E in a sector that typically trades at 28x looks cheap; the same 20x in a sector that typically trades at 16x looks expensive. This is the same comparable-company logic used to build a full comps set, as covered in Case 33: What Is a Valuation Multiple?
The Rate Environment: Why Multiples Aren't Static
The adjustment candidates most often forget is the interest rate environment. A valuation multiple is, at its core, a shorthand for a discounted cash flow calculation — investors are implicitly discounting a stream of future earnings back to the present. When the risk-free rate rises, the present value of cash flows far in the future falls more than the present value of cash flows arriving soon, because distant cash flows are compounded down over more years. This means high-growth companies, whose value is weighted toward the future, are structurally more sensitive to rate increases than low-growth, cash-generative businesses. A "fair" P/E in a 2% rate environment is not the same as a "fair" P/E in a 5% rate environment, even for the exact same company.
Putting It Together
A strong answer to "is 20x P/E expensive" never stops at the number. It works through growth (via the PEG ratio), industry context (via relative P/E), and the current rate environment (via the multiple's sensitivity to the discount rate) before reaching a conclusion — and it's comfortable saying the conclusion is different for different companies even at the identical headline multiple. If you want to practice applying this exact framework with real numbers, work through Case 34, which compares a 25% (0.25) growth software company against a 5% (0.05) growth utility, both trading at 20.0x — and see how differently the same multiple should be read once growth, industry, and rates are layered in. It also builds directly on the enterprise-value fundamentals in Case 31: What Is Enterprise Value?, which is worth reviewing first if multiples and EV bridges still feel shaky.