The P/E ratio: a question, not an answer

The price-to-earnings ratio is the most quoted number in investing and the most misunderstood. People treat it like a verdict — P/E of 12, cheap, buy; P/E of 60, expensive, avoid — as if a single figure could settle the matter. It can't. The P/E is not an answer. It's a question in disguise, and the whole skill is learning to hear the question it's actually asking.

What the P/E literally is

The arithmetic is trivial. Take the price of one share and divide it by the company's earnings per share — its annual profit split across all the shares. If a share trades at ₹300 and the company earned ₹15 of profit per share last year, the P/E is 300 ÷ 15 = 20. That's it. The hard part isn't the sum; it's knowing what the 20 means.

Here is the most useful way to read it. A P/E of 20 says: at today's price and today's profit, you are paying twenty rupees for every one rupee of annual earnings. Flip that around and it becomes vivid — if profit stayed exactly flat forever, it would take 20 years of earnings to pay back your purchase price. A P/E of 60 means a 60-year payback at current profits. A P/E of 8 means eight years. Suddenly the numbers stop being abstract. You'd happily wait 8 years for your money back from a sturdy business; you'd want a very good reason to wait 60.

Earnings yield: the same coin, flipped

Now turn the ratio upside down. Instead of price ÷ earnings, do earnings ÷ price. That's the earnings yield, and it's expressed as a percentage. A P/E of 20 is an earnings yield of 1/20 = 5%. A P/E of 10 is a yield of 10%. A P/E of 50 is a yield of just 2%.

This flip is quietly powerful because it puts a stock on the same footing as everything else you could do with your money. If a stock offers a 5% earnings yield, you can immediately compare it to what a government bond or a fixed deposit pays. When safe instruments yield 7%, paying for a 2% earnings yield (P/E 50) means you are betting heavily that those earnings will grow fast — because as a static number, 2% is a poor deal against a risk-free 7%. The earnings yield turns a vague 'feels expensive' into a concrete 'expensive compared to what?'

Trailing vs forward: which earnings?

There's a fork in the road the moment you compute a P/E: which year's earnings go in the denominator? You have two choices, and they tell different stories.

• Trailing P/E uses the profit the company actually earned over the last twelve months. It's based on facts — real, reported, audited numbers. Its weakness is that it looks backwards, and a business can change a lot in a year.
• Forward P/E uses estimated profit for the year ahead, usually from analysts. It tries to look forward, which is what investing is about — but estimates are guesses, often optimistic ones, and they get revised.

For a fast-growing company the two can look dramatically different. Imagine a firm at ₹600 a share. Last year it earned ₹10 per share, so trailing P/E is 60 — eye-watering. But it's growing fast and is expected to earn ₹20 next year, making the forward P/E 30. Same price, same company, very different first impression. Neither number is 'the truth'; one is fact looking back, one is hope looking forward. A careful investor checks both and asks whether the forward estimate is plausible or just a salesman's brochure.

Why a high P/E isn't 'bad' and a low one isn't 'good'

This is the heart of the chapter, so slow down here. A P/E is not a price tag; it's an encoded expectation. The market sets it by guessing the future, and the number is the crowd's collective bet written in shorthand.

A high P/E encodes optimism. When a stock trades at 50 times earnings, the market is effectively saying: we expect this company's profits to grow a lot, for a long time, with little risk of stumbling. The high price is the crowd pre-paying for a bright future. That bet can be completely justified — some businesses really do compound earnings at a blistering pace for years. But it can also be a fantasy, and the higher the P/E, the more perfection is already baked into the price, and the less room there is for disappointment.

A low P/E encodes pessimism — or risk. When a stock trades at 6 times earnings, the market is saying: we don't trust these earnings to last, or to grow. Maybe the industry is dying, maybe debt is dangerous, maybe profits are at a cyclical peak about to roll over. A low P/E is not a free gift; it's the market pricing in trouble it thinks it sees. Sometimes the market is wrong and the pessimism is overdone — that's where bargains live. But sometimes the market is right, and the cheap stock keeps getting cheaper. We'll spend the whole next chapter on exactly this trap.

Sanity-checking a P/E against growth and quality

If a P/E is a bet on growth, the obvious sanity-check is to weigh the P/E against the growth you actually believe in. A rough, widely-used instinct: a P/E can be reasonable if it's roughly in line with the company's sustainable earnings growth rate. A business growing profits ~25% a year carrying a P/E around 25 is at least internally consistent. The same P/E on a business growing 5% a year is a much harder sell — you're paying a growth price for an income-grade business.

But growth alone isn't enough; you have to weigh quality too. Two companies can both grow 20%, but one does it while earning fat returns on the capital it employs and the other does it by constantly raising money and piling on debt. The first kind of growth is worth paying up for; the second can be a mirage that evaporates the moment financing dries up. A high P/E on a genuinely high-quality compounder is a different animal from the same P/E on a fragile one — even though the number is identical. This is precisely why the ratio is a question: it points you towards the investigation, it doesn't replace it.

When earnings lie: the P/E's blind spots

The P/E has a fatal weakness baked into its 'E'. It is only as honest as the earnings beneath it. And earnings, as we'll explore in the financials, are partly an accounting opinion. A few situations break the ratio entirely:

• No profit, no P/E. A loss-making company has no meaningful P/E at all — the ratio is undefined or negative. Plenty of growing companies genuinely lose money for years, and the P/E simply can't speak about them.
• One-off profits. If a company sold a building or a subsidiary this year, its 'E' is puffed up by a gain that won't repeat. The P/E looks low, but only because the earnings are temporarily fat.
• Cyclical peaks. For commodity, metal or auto businesses, profits boom and bust with the cycle. The P/E looks lowest exactly when profits are at their unsustainable peak — the most dangerous time to buy. This is the cruel trap of cyclicals.

Price-to-book: when earnings aren't the right lens

Sometimes earnings are a poor anchor, and you need a different ratio. The main alternative is price-to-book (P/B), which compares the share price to the company's book value per share — roughly, the net worth on the balance sheet (what it owns minus what it owes), divided across all shares. A P/B of 1 means you're paying exactly the accounting net worth; a P/B of 3 means three times it.

P/B comes into its own for asset-heavy and financial businesses. For a bank, this is the natural lens: a bank's business is its balance sheet — loans, deposits and capital — and its profits are notoriously lumpy because a bad year can mean huge provisions for loans gone sour. Earnings can swing violently while the underlying book grinds along, so investors anchor on P/B and on the quality of that book. The same logic applies to insurers, NBFCs and heavy-asset industrials where the worth sits in tangible assets, not in a smooth earnings stream.

So both ratios are lenses, each clear for some businesses and blurry for others. The P/E asks how much am I paying per rupee of profit, and what does that price assume? The P/B asks how much am I paying per rupee of net worth, and is that worth real? Neither hands you a verdict. Both, used as questions, point you to where the real work is. In the final chapter we put them to the test against the most expensive mistake in valuation: confusing 'cheap' with 'good'.