The price-to-earnings ratio answers one question: how many dollars are you paying for one dollar of annual profit? A P/E of 20 means $20 of price for $1 of earnings.
Trailing vs. forward
| Type | Earnings used | Catch |
|---|---|---|
| Trailing P/E | Last 12 months (reported) | Backward-looking; can be stale after a big change. |
| Forward P/E | Next 12 months (estimated) | Forward-looking; only as good as the estimate. |
High or low — compared to what?
A P/E means nothing in isolation. A high multiple can be justified by fast, durable growth; a low one can be a bargain or a warning that earnings are about to fall. P/E is only useful against a peer, the company's own history, or its growth rate.
The value trap: a stock on a P/E of 6 looks cheap until you notice profits are shrinking. If earnings halve, that 6 quietly becomes a 12 — and the ‘cheap’ stock was expensive all along.
When P/E breaks down
P/E is useless when earnings are negative or tiny, and distorted by one-off gains and losses. For loss-making or cyclical companies, lean on sales multiples (P/S), cash-flow measures, or a normalized earnings figure instead.
