What Is a P/E Ratio, and Why It's Never the Whole Story

Open any stock page on a broker app or a financial news site in India and one number appears almost every time: the P/E ratio. Commentators say a stock 'trades at 40 times earnings' or that the NIFTY 50 'looks expensive at its current P/E'. The ratio is useful, but it is also one of the most misread numbers in investing. This explainer covers what it measures, how to calculate it yourself, and why two companies with the same P/E can be completely different propositions.

The calculation

The price-to-earnings ratio divides a company's share price by its earnings per share (EPS). EPS is the company's net profit attributable to shareholders divided by the number of shares outstanding. If a company earned ₹500 crore last year and has 50 crore shares, its EPS is ₹10. If the share trades at ₹250, its P/E is 25.

You get the same answer by dividing the company's total market value (market capitalisation) by its total annual profit. A P/E of 25 means the market is valuing the business at 25 times one year of its current profit. Another way to see it is through the inverse, the earnings yield: 1 divided by 25 is 4%. For every ₹100 you pay for the share, the company currently earns about ₹4 in profit on your behalf. Some of that may be paid out as a dividend; the rest is retained in the business.

Trailing, forward, standalone, consolidated

The P/E you see quoted depends on which earnings figure sits underneath it, and Indian data sources are not always consistent.

• Trailing P/E (often labelled TTM, trailing twelve months) uses the profit from the last four reported quarters. It is based on actual, audited or reviewed numbers, but it looks backwards.
• Forward P/E uses analysts' estimates of next year's earnings. It looks ahead, but estimates can be wrong, and they are often revised after results.
• Standalone earnings cover only the listed parent company. Consolidated earnings include subsidiaries. For groups with large subsidiaries, the two P/Es can differ sharply, so check which one a website is using before comparing.

When two sources show different P/Es for the same stock on the same day, it is usually because one is trailing and the other forward, or one is standalone and the other consolidated, not because either is wrong.

What a high or low P/E is really telling you

The share price reflects what investors expect a company to earn in the future, not only what it earned last year. So the P/E is best read as a summary of expectations. A high P/E usually means the market expects earnings to grow quickly, or to be unusually stable, or both. A low P/E usually means the market expects slower growth, more uncertainty, or a decline.

That is why a low P/E is not automatically a bargain and a high P/E is not automatically a bubble. The question is always whether the expectations built into the price are reasonable. A company on a P/E of 60 that keeps growing profit at 30% a year for a decade may turn out to have been cheap. A company on a P/E of 8 whose profits halve may turn out to have been expensive.

Five common traps

• The cyclical trap. Steel, cement, metals, sugar, and similar businesses earn their highest profits at the top of their cycle. Because earnings are at a peak, the P/E looks lowest exactly when the outlook may be about to turn. At the bottom of the cycle, depressed profits make the P/E look very high. Reading these stocks on P/E alone can lead to the opposite of the intended conclusion.
• One-off items. A large gain from selling a division or a land parcel inflates net profit for one year and makes the P/E look artificially low. A one-time write-off does the reverse. Check whether the year's earnings include exceptional items.
• Losses and tiny profits. A company that lost money has no meaningful P/E, and one that barely broke even can show a P/E in the hundreds. Neither number tells you much.
• Cross-sector comparisons. Banks, software companies, and utilities have different growth rates, capital needs, and risks, so their 'normal' P/E ranges differ. Comparing a private bank's P/E with an IT services company's says little. Comparisons are more useful within a sector, or against the same company's own history.
• Debt is invisible. The P/E looks only at equity value and profit after interest. Two companies with identical P/Es can carry very different amounts of debt, which changes how risky the earnings are.

Index P/E: the market as a whole

The same idea applies to an index. NSE publishes the P/E, price-to-book, and dividend yield of the NIFTY 50 and other indices, calculated from the earnings of the constituent companies. Market commentary often compares the current index P/E with its long-run average to argue that the market is 'cheap' or 'expensive'.

That comparison has some value as context, but it is a weak timing tool. Index P/Es can stay above or below their averages for years, the mix of companies in an index changes over time (an index heavier in banks and IT today may have been heavier in commodities a decade ago), and the method used to calculate index earnings has itself been changed in the past. Treat the index P/E as one description of the market's mood, not as a signal.

Related ratios worth knowing

• PEG ratio: the P/E divided by the expected earnings growth rate. A P/E of 30 with expected growth of 30% gives a PEG of 1. It tries to adjust the P/E for growth, but inherits all the uncertainty of the growth estimate.
• Price-to-book (P/B): share price divided by book value (net assets) per share. It is commonly used for banks and financial companies, whose assets are largely financial and marked closer to their value.
• EV/EBITDA: enterprise value (market value plus debt minus cash) divided by earnings before interest, tax, depreciation, and amortisation. It includes debt and is often used to compare companies with different capital structures.

A worked example

Imagine two hypothetical companies, each trading at ₹200 a share with EPS of ₹10, so both have a P/E of 20. Company A is a consumer goods maker whose profit has grown steadily by around 12% a year for a decade, with no debt. Company B is a commodity producer whose profit last year was the highest in its history because prices for its product spiked; it has significant borrowings.

On the screen, they look identical. In practice, the market is probably pricing Company A's earnings as durable, and Company B's earnings as a peak that may not repeat. If B's profit falls back to ₹5 next year, its P/E at the same share price jumps to 40. The ratio did not warn you; the context did.

How to use the P/E sensibly

• Check which earnings it uses: trailing or forward, standalone or consolidated.
• Compare a company with its own history and with close peers, not with unrelated sectors.
• Look at the earnings trend over several years, not just the latest year.
• Look at debt, cash flow, and return on capital alongside the P/E.
• Remember that the ratio summarises expectations. The useful question is whether those expectations make sense.

Used this way, the P/E is a good starting point for asking questions about a business. Used on its own, it is one of the easiest ways to talk yourself into a conclusion the numbers do not support.