The PEG ratio, made simple

What this lesson covers

• What the PEG ratio means, in plain English
• The basic PEG formula
• How the P/E ratio and expected earnings growth are connected
• A simple example of calculating PEG
• How PEG adds context to a company's P/E ratio
• Why a lower or higher PEG should not automatically be read as good or bad
• Why expected growth rates and forecasts need to be treated carefully

Where this sits in the series: Lesson 4 covered what a company earns. Lesson 5 covered what investors pay for those earnings. This lesson asks the obvious next question — how fast are those earnings expected to grow?

Quick recap: what P/E told us

The P/E ratio is the share price divided by earnings per share. It helped us ask one question: how much am I paying for the current earnings?

The main point of that lesson was that P/E is a starting point, not a final answer.

• A lower P/E does not always mean cheap.
• A higher P/E does not always mean expensive.

A company with a high P/E may still be reasonable if earnings are growing fast. A company with a low P/E may still be risky if growth is weak or declining. P/E is useful, but it looks mainly at today's earnings — or last year's.

The problem PEG tries to solve

P/E connects price and earnings. But investors usually care about the future.

PEG brings that third element into the conversation. It asks whether the price makes sense compared with expected growth — a growth-adjusted way of reading the P/E ratio.

It is not perfect. What it does well is stop us looking at P/E in isolation.

The PEG ratio formula

PEG stands for Price/Earnings-to-Growth. The formula is simple.

One important detail. Use the growth rate as a whole number, not as a decimal. A company expected to grow earnings by 20 per cent a year goes in as 20, not 0.20. Get this wrong and the answer will be a hundred times out.

Take a company with a P/E of 30, expected to grow earnings at 20 per cent a year. Thirty divided by twenty gives 1.5.

That result does not mean buy, and it does not mean sell. It gives you one clue about valuation compared with growth. Like P/E, the answer is a multiple rather than a dollar amount.

The four steps, in order

A simple process is enough.

A quick example: a P/E of 24 divided by growth of 12 gives a PEG of 2.0.

Step four is the one beginners skip, and it matters most. Do not stop at the number. Compare the PEG result with the company's quality, its risk, its industry, and how reliable that growth estimate actually is.

How to read PEG — a beginner guide

There is a common rough guide to interpreting the result.

These are rough signals, not strict rules. Notice the word "may" in all three. A PEG below 1 may suggest a company looks cheaper compared with its growth — or it may be telling you the market doubts that growth will arrive. The ratio cannot tell you which.

Same P/E, different growth

This is the comparison that shows why PEG is worth calculating at all. Two companies, identical P/E ratios.

On P/E alone these two companies look identical. They are not. Company A is expected to grow earnings three times faster, and once growth enters the calculation the two look very different.

The same P/E does not tell the whole story. Growth changes the interpretation — which is precisely the gap this ratio was built to fill.

Mini practice: read this one slowly

Now the other way around. Two companies with the same expected growth, but different P/E ratios.

On PEG alone, Company D looks cheaper relative to growth. Work the arithmetic yourself before moving on — 40 divided by 20, and 20 divided by 20.

But the final decision still needs business quality, risk and your own research. PEG alone is never the whole decision.

Why PEG can be helpful

• It makes the P/E ratio more meaningful, by attaching it to something.
• It helps compare growth companies against each other.
• It reminds us to ask about the future, not only the present.

PEG is especially useful when comparing companies with different growth rates — exactly the situation where P/E on its own is least informative.

It also carries a useful reminder: valuation is not only about the current number. It is about whether future earnings can justify the price being paid today.

Five common traps to avoid

PEG can mislead beginners badly if it is treated as a magic number.

• Trap 1. Using growth estimates as if they are guaranteed. They are forecasts, not facts.
• Trap 2. Comparing companies from completely different industries.
• Trap 3. Ignoring debt, margins, competition and business quality.
• Trap 4. Treating a low PEG as an automatic bargain. A low PEG can reflect risk, not just opportunity.
• Trap 5. Using one-year growth for a company with unstable earnings.

Trap 1 is the one worth sitting with. P/E is built from a share price you can look up and earnings a company has actually reported. PEG adds a third input that is not a fact at all — it is somebody's expectation, and the ratio gives you no way of seeing how sound it is.

PEG works best in context

The ratio is one part of a picture, not a substitute for one.

A good investor does not ask for one perfect ratio. A good investor builds a picture. Better valuation thinking comes from combining these pieces rather than hunting for a single number that settles the question.

A beginner checklist before using PEG

Five things to check before you trust the number.

• Is earnings growth positive and realistic?
• Am I comparing similar companies?
• Is the company profitable enough for P/E to matter?
• Is growth stable, or just one unusual year?
• Have I checked debt, margins and risks?

The point is not to make research complicated. It is to make sure a single number never carries a decision on its own.

The key takeaway

PEG helps you ask a better question: is the P/E reasonable for the growth I expect?

That is the whole value of it. It does not give you an answer. It improves the question you are asking about a company, which over time is worth considerably more.

Use PEG as a clue, not a conclusion.

Where to go next

If you'd like to see these ideas applied to real companies and real results, How Markets Think is where I write that up each week — earnings, deals and market moves, explained in plain English.