Current market price of share / EPS
The P/E tells you how many years you would have to wait to get your money back on your investment.
Example
So if shares are trading at £3.00 and the EPS is 60p, the P/E is 5.
But how meaningful is this? There is an argument that the P/E is a flawed concept because whilst EPS is based on historical data (actual earnings figures) the current market price of a share is based on future expectations. And since future expectations are only partly determined by past performance the logic of a connection between them is tenuous.
In determining whether a P/E is 'high' or 'low', compare it with the P/E for other companies in the same sector.