P/E Ratio — Questioning What "Cheap" Really Means¶
For / Key Points
For: Those who know the term P/E but have never used it beyond "low P/E equals cheap." Anyone who wants to use indicators for investment decisions but isn't sure how far to trust the numbers.
Key Points:
- P/E is not "payback period" — it's a price tag of expectations, reflecting how much market participants expect from a company's future
- A P/E of 15 means different things depending on industry, growth rate, and interest-rate environment. Comparing numbers without context is meaningless
- Knowing when P/E breaks — money-losing companies, cyclicals, buybacks — is the prerequisite for using it well
"A P/E of 15 — that's cheap." Start investing and you'll hear this almost immediately.
But as the previous article established, stock prices are the outcome of participant consensus, not an objective reflection of corporate value. "Cheap" and "expensive" turned out to be relative concepts whose meaning shifts depending on the reference point and the observer.
P/E is the most widely used indicator for gauging that "cheap or expensive" question. That's exactly why you need to understand precisely what it measures — and what it doesn't.
What P/E Actually Is — Deconstructing the Formula¶
The P/E formula is straightforward:
Suppose a company earns ¥10 billion in net income with 50 million shares outstanding. ¥10 billion ÷ 50 million = ¥200. That's the Earnings Per Share (EPS) — the company's total profit allocated to a single share.
If this company's stock trades at ¥3,000, the P/E is 3,000 ÷ 200 = 15. This is commonly explained as "the stock is priced at 15 years' worth of earnings."
But it doesn't mean you'll recoup your investment in 15 years. EPS fluctuates annually, and nothing guarantees that today's profit level persists for 15 years. What the formula actually captures is how much expectation market participants are placing on the company's future earnings.
A P/E of 15 signals that participants have agreed to pay 15 times the current earnings level. A P/E of 30 means they're pricing in greater future growth. A P/E of 8 implies low growth expectations or some risk baked into the price.
In other words, P/E is not "payback period" — it's a price tag of expectations. Compare numbers without this recognition and you'll misjudge.
So how should you compare P/E numbers?
Is "P/E of 15 Means Cheap" Actually True?¶
The absolute value of P/E — whether it's 15 or 30 — tells you nothing about cheapness on its own. Three main factors determine where P/E "should" sit.
Industry differences. JPX (Japan Exchange Group) publishes sector-level P/E data. On the Prime Market, utilities (electricity and gas) typically show single-digit P/Es, while information and communications companies regularly exceed 20x. Utilities deliver stable but slow-growing earnings. Tech and telecom command higher P/Es because participants expect growth.
A P/E of 15 looks "cheap" within the information and communications sector. In utilities, it might be "somewhat expensive."
Growth-rate differences. A company growing earnings by 30% annually at a P/E of 30 versus a flat-earnings company at a P/E of 15 — which is "more expensive"? The PEG ratio (P/E ÷ earnings growth rate, using the percentage figure directly) adjusts for this.
At P/E 30 with 30% growth, PEG is 1.0. At P/E 15 with 5% growth, PEG is 3.0. On a PEG basis, the high-P/E growth company can look "cheaper."
Interest-rate environment. When rates are low, bond yields decline, making equities' future earnings relatively more attractive. The result: market-wide P/Es rise. Japan's extended low-rate policy means the overall P/E level has fluctuated substantially over time.
Comparing today's P/E to "the historical average" falls apart if the interest-rate environment has changed.
P/E comparison always requires context: "compared to what?" and "under what conditions?" Calling a stock cheap based solely on its P/E number is like saying someone who's 170 cm tall is "tall." Among basketball players, they're short. In an elementary school, they're tall.
There's another layer of caution when comparing P/Es: what goes into the denominator.
Trailing P/E vs Forward P/E — Which Should You Use?¶
P/E comes in two flavors. Trailing P/E uses the most recent confirmed earnings as the denominator. Forward P/E uses analysts' or management's projected earnings for the coming period.
| Trailing P/E | Forward P/E | |
|---|---|---|
| Denominator | Last 12 months' confirmed EPS | Next-period projected EPS |
| Strength | Numbers are finalized and objective | Reflects future earning power |
| Weakness | Backward-looking; doesn't capture the future | Falls apart if forecasts miss |
Brokerage apps and screening tools sometimes display "P/E" without specifying which type. The same stock can show a trailing P/E of 20 and a forward P/E of 15. If earnings growth is expected, forward EPS will be larger, pushing forward P/E below trailing P/E.
The bottom line: the stock market is fundamentally forward-looking. Participants price stocks based on future earnings, not past ones. For investment decisions, forward P/E tends to be closer to reality.
However, forward P/E carries a "whose forecast?" problem. Company-issued guidance tends to be conservative. Japanese companies in particular are known for understating earnings forecasts and then issuing upward revisions mid-year. Analyst consensus (the average of multiple analyst estimates) aggregates better, but for small- and mid-cap stocks with thin analyst coverage, the sample skews.
When using forward P/E, make it a habit to check the assumptions behind the forecast. Declaring a stock "cheap at a forward P/E of 15" without examining the underlying assumptions falls into the same trap as with trailing P/E.
So far we've covered how to use P/E. Next: situations where P/E stops working altogether.
When P/E Doesn't Work¶
P/E is not universal. Its formula structurally fails under certain conditions.
Money-losing companies. When EPS turns negative, P/E goes negative too. "P/E of −20" is useless as a comparative metric. For loss-making companies, you need earnings-independent measures: PSR (price-to-sales ratio), which compares stock price to revenue, or DCF (discounted cash flow) analysis based on projected future cash flows.
Cyclical stocks. In cyclical industries — steel, shipping, chemicals — P/E moves counterintuitively. During a boom, swelling profits push EPS up and P/E down. Buying because "P/E of 5 looks cheap" can mean buying right before a downturn slashes earnings and sends P/E soaring.
For cyclicals, low P/E often marks the peak; high P/E often marks the bottom.
Hyper-growth stocks. Rapidly growing companies often reinvest most of their earnings. Accounting profit stays suppressed, producing extreme P/Es of 100 or 200.
In this case, the high P/E doesn't signal "overvalued" — it signals "most of the earnings are deferred into the future." Judging by P/E alone means potentially missing promising companies in their early growth phase.
Knowing where P/E fails is itself the prerequisite for using P/E properly. Buying because "P/E is low" without recognizing the indicator's limits is like estimating distance on a map without checking the scale.
Even where P/E does work, one more concern remains: what if the denominator — EPS itself — is distorted?
P/E Traps — Questioning the Quality of Earnings¶
EPS, the denominator of P/E, can swing dramatically due to corporate actions or one-off events. A low P/E doesn't always signal "cheap."
Buyback-inflated EPS. When a company repurchases its own shares, the outstanding share count drops. Total earnings stay the same, but EPS rises because the pie is split among fewer shares — and P/E falls accordingly.
This isn't an improvement in business profitability; it's arithmetic from share-count reduction. Reading a buyback-driven P/E decline as "cheap" jumps to conclusions.
Special gains and losses. Real-estate sale proceeds, lawsuit settlements, subsidiary divestitures — one-time items like these distort EPS when included. A large special gain inflates EPS for that period, making P/E look artificially low. When the gain disappears the following period, P/E jumps back up.
Accounting-standard differences. Japanese GAAP, IFRS, and US GAAP differ in how they handle goodwill amortization and R&D capitalization. The same business activity produces different EPS — and therefore different P/Es — depending on the accounting standard applied.
The treatment of goodwill amortization between Japanese GAAP and IFRS, in particular, can create substantial profit differences. Comparing P/Es across companies using different standards requires this adjustment.
When looking at P/E, verify whether the denominator reflects "sustainable earnings." Trusting a P/E built on temporarily inflated profits means a seemingly rational conclusion may be built on sand.
How to Actually Use P/E — In Context, Not in Isolation¶
Synthesizing the discussion, the right way to use P/E emerges. P/E is not a standalone indicator but a tool that functions only when placed in context.
Compare within the same industry. P/E comparisons belong between companies in the same sector with similar business models. Cross-industry P/E comparisons are largely meaningless.
Compare against the company's own history. Look at the current level relative to the company's average P/E over the past five years. But if the business mix has changed or the interest-rate environment has shifted, historical levels become reference points at best.
Combine with other indicators. PBR (price-to-book ratio) measures how the stock price compares to the company's net asset value. ROE (return on equity) measures profitability relative to shareholders' equity. These three indicators are linked mathematically:
Note: ROE is entered as a decimal (10% ROE = 0.10). The periods and definitions (trailing vs. forward, diluted EPS, etc.) must align for the equation to hold.
If P/E is low but ROE is also low, PBR may show the stock is "fairly valued." Conversely, if P/E is high but ROE is exceptionally high, PBR may stay within an acceptable range. What one indicator hides, combining multiple indicators can reveal.
The relationship among P/E, PBR, and ROE will be explored in dedicated articles later in this series.
Takeaways¶
- P/E is a "price tag of expectations," not an absolute measure of cheap or expensive
- The "appropriate P/E" shifts with industry, growth rate, and interest-rate environment. Context is required for comparison
- Forward P/E is closer to market reality, but using it without checking the forecast's assumptions is risky
- P/E structurally fails for money-losing companies, cyclicals, and hyper-growth stocks
- Buybacks, special gains, and accounting-standard differences distort EPS and undermine P/E reliability
- P/E works best in combination with PBR, ROE, and other indicators — not alone
The previous article stated that no "fair price" exists. P/E reinforces that recognition. What P/E shows is "how much expectation participants are placing right now" — not "what this stock should be worth."
An indicator is a tool for decision-making, not the decision itself. Whether you hold this distinction determines whether you end up controlled by indicators or in control of them.