Recently, I found that many people are still a bit confused about the price-to-earnings ratio (P/E ratio). Actually, understanding this concept is very helpful when choosing stocks. In simple terms, the P/E ratio is the stock price divided by earnings per share, representing how many years it takes to recover your investment cost through the company's profits.

Let's take a real example: if a stock's price is 520 yuan and its earnings per share are 39.2 yuan, then the P/E ratio formula is 520 divided by 39.2, which is approximately 13.3 times. This means it would take about 13 years to break even. A lower P/E ratio usually indicates the stock is relatively cheap, but you also need to consider industry characteristics, because the reasonable P/E ratio varies greatly across different sectors.

Regarding how to calculate the P/E ratio, there are actually several different methods. The most common is using last year's annual earnings per share, called the static P/E ratio. Another method is summing up the earnings of the latest four quarters, called the rolling P/E ratio, which can more promptly reflect the company's recent performance. There is also an estimated P/E ratio, but honestly, this indicator is harder to guarantee accuracy because different institutions have different forecasts.

How to judge whether a P/E ratio is high or low? I usually use two methods. The first is comparing with companies in the same industry, using similar enterprises for comparison. The second is looking at the company's own historical trend, comparing the current P/E ratio with the past five years' performance to see if it is at a high, low, or medium level.

In practice, there is a very useful tool called the P/E ratio river chart, which plots several lines on the stock price chart representing the historical highs, lows, and the stock prices corresponding to various P/E multiples. This way, you can quickly see whether the current stock price is overvalued or undervalued.

But be aware that a low P/E ratio does not necessarily mean the stock will rise, and a high P/E ratio does not necessarily mean it will fall. Often, the market is willing to assign a high valuation because it is optimistic about the company's future growth. So, even if tech stocks have very high P/E ratios, their prices can still keep rising.

The P/E ratio does have some limitations. First, it only considers equity value and does not account for the company's debt situation. Companies with high or low debt levels can have the same P/E ratio but vastly different risks. Second, it is difficult to accurately determine whether a P/E ratio is high or low because a high P/E might be due to temporary profit declines with solid company fundamentals, or because the market is optimistic about future growth and has priced it in early. Lastly, startups or companies that are not yet profitable cannot be evaluated using the P/E ratio at all; in these cases, other indicators like price-to-book ratio or price-to-sales ratio should be used.

If you want to use the P/E ratio to guide your investments, the key is to understand its formula and application scenarios, rather than treating it as the sole criterion. Combining it with other indicators and understanding the company's fundamentals will help you make more prudent decisions.