The forward P/E is based on what the company is expected to earn a year in the future. And the trailing P/E is based on what the company earned in the previous year. The difference between a trailing and forward P/E can be dramatic.
Which P/E should you pay attention to? The answer is, both. It's especially important now to consider both because future earnings are so unpredictable.
Rather than trying to pick which P/E ratio to use, compare them with each other. If you see a big difference between the two, you might learn more about the company's valuation if you find out why the forward and trailing P/E are so different.
For instance, if you notice that a stock has a high double-digit trailing P/E and a low forward P/E, you know that analysts are predicting earnings to recover sharply, but investors aren't believing the estimates yet. In that situation, if the company delivers as analysts hope, there could be a nice upside for the stock.
Similarly, if a stock's trailing P/E is low but the forward P/E is high, it might be a clue that analysts are being extremely bearish about the company's future, which might be a warning to you. On the other hand, this situation might mean investors are being overly optimistic about a stock's future, and they could be in for a rude awakening.