The price to earnings (P/E) ratio is a common formula used in the analysis of stocks and stock markets. It tells you the amount investors are willing to pay for a dollar of reported profits. There are several different ways the a P/E ratio can be calculated. Below are the 3 most commonly used calculations.

### 1. P/E ratio based on last year’s earnings

This calculation takes the current price of a stock divided by last year’s corporate earnings.

Here's how it works.

Let's look at a company who's stock price is $50.00 per share. Let's assume the company is making a profit and reported earnings of $5.00 per share. The P/E would be $50.00 / $5.00 or a 10 to 1 ratio. This means at the current price investors are willing to pay $10 for every $1 of reported earnings.

The problem with relying on this calculation is the possibility that next year will be nothing like last year; corporate earnings could me much higher, or much lower.

### 2. P/E ratio based on a forecast of earnings

This calculation takes the current price of the stock (or group of stocks) divided by an average of all of the predicted earnings put forth by analysts and the companies themselves. The problem with this calculation is there are numerous occasions where companies do not earn what they were projected to earn.

That guessing game is what makes stocks and the stock market so difficult to evaluate.

No one truly knows what will happen next year. Professional investors or investment management companies will make forecasts; some will be right and some will be wrong.

At the end of the day, analyst opinions and forecasts will always be changing as new information becomes available.

When you look at stock research websites, they will normally provide you with ratios based on past and projected earnings.

This gives you a frame of reference to use when comparing one company to another company that is in the same industry. If two similar companies have vastly different P/E ratios you'll want to do more research to find out why.

### 3. P/E Ratio based on a ten year average of earnings

This calculation is most often used to look at the value of an entire market instead of an individual stock. It takes the current price of the market divided by corporate earnings as averaged over the past ten years. This ratio is called P/E 10 and the method was developed by Professor Robert Schilling. It is designed to even out the inconsistencies that can come from using only one year of past or projected earnings' data.

Much research supports the validity of P/E 10 as an appropriate way to value a market as a whole. It can be useful in figuring out where things are relative to the past. This data is often used to make a determination of how "expensive" the market is.

When running statistics on the stock market it is often best to use all three types of P/E ratios - not just one.

Regardless of which way you look at P/E ratios, the information is useless unless you can put it in perspective. For a better understanding of how to use P/E ratios read Putting Some Perspective on the P/E Ratio of the S&P 500.