What Is the P/E Ratio? Price-to-Earnings Explained

The price-to-earnings ratio — the P/E — is the most quoted number in equity investing. It tells you how much investors are willing to pay for every unit of a company’s current earnings.

Once you can price a whole company with market cap, the natural next question is: is it worth it? The P/E ratio is the quickest way investors size that up.

The formula

The P/E ratio compares a company’s share price to its earnings:

P/E = share price ÷ earnings per share (or, equivalently, market cap ÷ total net income).

A P/E of 20 means investors are paying £20 for every £1 of the company’s annual earnings. On its own, though, a P/E is just a number. It only means something in comparison — to the company’s peers, to its own history, or to its expected growth.

Forward vs trailing P/E

There are two versions you’ll see quoted:

Trailing P/E uses the last 12 months of actual, reported earnings.
Forward P/E uses analysts’ estimates of future earnings — typically the next 12 months.

For a growing company, the forward P/E is usually lower than the trailing one, because earnings are expected to rise — the same price is divided by a bigger future number.

Why do some companies have very high P/Es?

A high P/E is not the same as "expensive." It usually signals that the market expects strong future earnings growth, and so is willing to pay more for each pound of today’s earnings. This is why many technology companies trade on high P/Es — their price is anchored to the profits investors expect years from now, not just what they earn today. A low P/E, equally, can mean a company is cheap or that the market expects its earnings to shrink.

What the P/E doesn’t tell you

The P/E is popular because it’s simple, but that simplicity hides a lot. It says nothing about:

How fast earnings are growing.
How much debt the company carries.
The quality of those earnings, which one-off items can distort.

It also isn’t comparable across very different industries. That’s why analysts rarely stop at the P/E — they pair it with other measures like the growth-adjusted PEG ratio, EV/EBITDA, or the balance sheet itself.

A quick trap: the negative P/E

If a company is loss-making, its earnings are negative — which makes its P/E negative too. A negative P/E is generally treated as "not meaningful": you simply can’t value a company sensibly on it. For unprofitable firms, analysts switch to other metrics entirely.

Key takeaways

P/E = share price ÷ earnings per share — what you pay per unit of earnings.
It only means something in comparison to peers, history, or growth.
A high P/E signals expected growth, not simply "expensive".
It ignores growth, debt, and earnings quality — never use it alone.

What is the P/E ratio?