P/E ratio: what it tells you – and when it fools you
The P/E ratio divides a stock's price by its earnings per share. It tells you how many years of profit you pay for one share. It is quick to calculate, but sensitive: a single special item in earnings can double or halve it, even though nothing about the company has changed. So always check whether the earnings show a normal year.
What the P/E ratio measures
The price-to-earnings ratio, P/E for short, is the best-known figure on the stock market. It compares two things: what you pay for a share, and what the company earns per share.
A P/E of 20 means you pay twenty times one year's earnings today. Put simply, the company would need 20 years to earn its purchase price if earnings stayed the same. Read the other way round, that is an earnings yield of 5%: every year the company earns 5% of what you paid.
The formula
- Share price
- the current price of one share
- Earnings per share
- profit of the last twelve months, divided by the number of shares
Most of the time the calculation uses earnings of the last twelve months. That is a measured value. There is also the forward P/E, which uses the estimated earnings of the next year. That is an assumption, not a measurement – and that is exactly how you should read it.
The worked example
Let's take our made-up Example Inc. again. Its share costs €100, and in a normal year it earns €5 per share. Now we look at the same company in three different years: once normal, once with a one-off charge, once with a one-off gain.
| Year | Earnings per share | P/E |
|---|---|---|
| Normal year | €5.00 | 20.0 |
| One-off tax charge (−€2) | €3.00 | 33.3 |
| Sale of a division (+€5) | €10.00 | 10.0 |
In the second case the stock suddenly looks expensive, in the third like a bargain. In reality nothing about the business has changed. The P/E ratio simply calculated honestly what was in the earnings – and there was a special item in them.
A real special item
Meta shows that this is not just a thought experiment. In the third quarter of 2025 the company booked a one-time income tax charge of $15.93 billion, triggered by a change in US tax law. No cash left the company for it; the charge was non-cash. Earnings per share for the quarter came in at $1.05 instead of $7.25.
Anyone who then calculates the P/E from the last twelve months of earnings gets a value that is far too high for a whole year. On paper the company looked more expensive than it was. Only once that quarter drops out of the twelve-month figure does the P/E become accurate again.
How to check whether the P/E is right
Read the explanation of earnings. Companies name large one-off effects in their quarterly report, often in the very first announcement. Words like "one-time", "special item" or "impairment" are a signal.
Compare with operating income. Tax charges or asset sales usually don't show up in earnings before interest and taxes. If only net earnings jump but operating income doesn't, there is almost always a special item behind it.
Look at several years. An average over three to five years smooths out outliers. A single year can deceive, a series hardly.
Add free cash flow. What really stays in the till is hard to bend with bookkeeping. If earnings differ a lot from free cash flow, it pays to look closer.
What to compare the P/E with
A P/E ratio on its own says little. Comparison makes it useful: with the company's own history and with similar companies. A fast-growing software company almost always has a higher P/E than a utility – that is not a mistake, but the price of more expected growth. How to judge the two together is shown in the article Valuing growth.
When a company makes a loss, the P/E is meaningless. For banks and REITs it only fits to a limited extent: banks are better compared by book value, REITs by funds from operations (FFO).
Three common mistakes
See at a glance whether a year is out of line.
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Frequently asked questions
What is a good P/E ratio?
There is no fixed threshold. Whether a P/E is high or low only becomes clear when you compare it with the company's own history and with similar companies.
What does a negative P/E mean?
The company made a loss. A negative P/E makes no sense and is usually not shown at all. Other figures help then, such as revenue or free cash flow.
Trailing or forward P/E – which is better?
They complement each other. The P/E based on the last twelve months is measured; the forward P/E is based on estimates. Estimates can be wrong – so read the forward P/E as an assumption.
Does the P/E ratio work for banks and REITs?
Only to a limited extent. For banks, book value is more meaningful; for REITs, funds from operations (FFO), because depreciation distorts their earnings.
Not investment advice. All content is for informational and educational purposes only. Example Inc. is made up; real companies are mentioned only as examples, not recommendations. Meta figures: quarterly results release of Oct 29, 2025.