The price-to-earnings ratio is share price divided by earnings per share. If a stock trades at 20 and earns 1 per share, its P/E is 20. That is the entire calculation — the difficulty is in the interpretation.
What it actually means
A P/E of 20 means you are paying 20 for each 1 of annual profit. Flip it upside down and you get the earnings yield: 1/20 = 5%. That framing is more useful, because it lets you compare a stock against a bond yield or a savings rate directly.
Low P/E is not the same as cheap
This is the trap that catches most beginners. A low P/E means the market expects the earnings to fall. Sometimes the market is wrong and you have found a bargain; often it is right and you have found a business in decline — a value trap. A high P/E, equally, means the market expects growth, and if that growth arrives the stock was cheap all along.
Three things that break the ratio
- Cyclical companies. Miners, carmakers and chemical firms show their lowest P/E at the top of the cycle, when profits are peaking and about to fall. For cyclicals, a low P/E is a warning, not an invitation.
- No earnings. A loss-making company has no meaningful P/E at all. The ratio silently stops working exactly where beginners most want a number.
- Debt. P/E ignores the balance sheet entirely. Two companies can show the same P/E while one is debt-free and the other is one bad quarter from trouble.
How to use it properly
Compare a company against its own history and against direct competitors — never against the whole market. A software firm and a utility have different P/Es for good structural reasons, and "the utility is cheaper" is not an insight.
Summary
P/E is a question, not an answer. It tells you what the market expects; your job is to decide whether those expectations are wrong. If you cannot articulate why the consensus is mistaken, a low P/E is not an edge.