Trying your hand at the stock market? Chances are, you’ve come across the term “P/E ratio”. If you’re like many who are new to the stock market, you’ve looked at this phrase and asked yourself, “What in the world is that?”

P/E ratio, otherwise known as the price-to-earnings ratio, is a formula that investors use to determine the value of a company’s share. It is one of the most common formulas used to determine the value of a stock. The formula compares the price of a company’s share to the earnings per share (EPS) of the company in order to determine how much an investor is paying for $1 of the company’s earnings. Let’s take a deeper dive into the P/E formula. Use the links below to jump ahead to a section of your choosing.

- P/E Formula and Calculation
- Understanding P/E Ratio
- Limitations of the P/E Ratio

## P/E Formula and Calculation

First thing’s first: let’s learn the price to earnings ratio formula and how to calculate it. The price-to-earnings ratio formula is as follows: the price of a single share of a company’s stock (What is a stock?), divided by the company’s earnings per share (EPS). The ratio of these two variables will tell you exactly how much an investor is spending for a single dollar of the company’s earnings.

Finding the cost of a company’s stock is extremely simple. In order to find the price of a single share of a company’s stock, all you need to do is enter the company’s stock ticker symbol (the series of characters that represents that company on the stock market) into a finance website, such as investor.gov. You’ll quickly find the current cost for a single share of that company’s stock. Google also keeps an up-to-date Market Summary for the prior day’s stock market, so a quick Google search will often bring exactly the answer you’re looking for.

Determining a company’s earnings per share (EPS) can be a bit trickier. Earnings per share are broken down into 2 categories: trailing earnings and forward earnings. Trailing earnings, often shortened to TTM, are the company’s core earnings over the trailing, or prior, 12 months. This number is the profit that the company has generated over the past 12 months of business. Remember that we’re talking about the net income of a business, rather than the gross income (Need a refresher? Learn more about gross income vs net income.). P/E ratios calculated with trailing earnings are known as the trailing P/E (P/E TTM). Forward earnings, on the other hand, are the predicted earnings that the company will generate over the next 12 months. P/E ratios calculated using forward earnings are known as the forward P/E. Both types of earnings are divided by the total number of public shares on the market in order to generate their EPS. More on this later.

Let’s try out an example. Say you’re looking to determine the trailing P/E of a fictional company AlphaBet Corporation, known on the stock market as ABC. Their share price is currently at $50 per share. Their trailing earnings per share is $5. Divide the $50 per share by the $5 EPS, and you’re left with a P/E of 10. This means that investors are paying $10 for every $1 in earnings per share.

## Understanding P/E Ratio

So, ABC has a P/E of 10. What does that mean for you?

In the most general sense, the lower a P/E ratio, the less an investor is paying for each dollar of a company’s earnings per share. A higher P/E ratio means that an investor is paying more per EPS. But, unfortunately, determining which stock to buy isn’t as simple as “look for the lowest P/E ratio”.

It is imperative to remember that everything on the stock market is relative. “Good” and “bad” numbers are different for each and every industry. An electronics company and an automotive company are functioning in two vastly different landscapes. Therefore, in order to determine what is a good price to earnings ratio, you’ll need to understand the landscape of P/E ratios in the industry. Look at similar companies’ P/E ratios to better understand the relative value of your company’s P/E ratio. If ABC’s price-to-earnings ratio seems extremely high as compared to other companies in the industry, it may be an overvalued stock. On the other hand, if it seems extremely low as compared to other companies in the industry, it may be a very valuable stock.

Let’s try another example. We’ve already determined that ABC’s price is $50 per share, earnings are $5 per share, and P/E is 10. A competitor, DOG, also has stock for $50 per share. Their earnings, on the other hand, are $2 per share, making their PE 25 (50/2=25). An investor would pay $10 for every $1 of ABC’s earnings per share, but they’d have to pay $25 for every $1 of DOG’s earnings per share. With a better understanding of the landscape, we can see how ABC sits relative to its competitors.

A company’s price to earnings ratio may also be looked at relative to itself. Remember those two types of earnings we reviewed earlier? We can compare a company’s trailing P/E to their forward P/E to better understand the value of a stock. A company with a high trailing P/E ratio may have been rather unprofitable the prior 12 months because theywere preparing to ramp up business substantially, and took on a number of upfront costs. They may be expecting a boom of profits over the forward 12 months, leaving them with a substantially lower forward P/E. By reviewing these numbers in comparison to each other, we may see an opportunity for a long-term investment.

## Limitations of the P/E Ratio

While the price to earnings ratio is certainly one of the most widely used calculations among stock market investors and analysts, it’s not a cut and dry way to determine a good or bad stock. It gives investors a good understanding of the value of stock in a particular moment, but it certainly has its short-comings.

Just as the stock market is relative, it’s also in a constant state of fluctuation. It is re-evaluated and recalculated constantly. Why does this matter when it comes to the price to earnings ratio? Well, just look at the variables we use to determine the P/E ratio.

First, we have the “price” of the price-to-earnings ratio: the cost of a single share of a company’s stock. Stock prices fluctuate every single day based on supply, demand, current events, and more. Typically, the cost of a company’s stock will be reported as the cost that it was when the stock market closed the prior day. Each time a company’s stock price changes, their P/E ratio will change. Certain companies may tend to have a greater fundamental volatility than others, leaving their stock price changing substantially each and every day. Even those with low fundamental volatility experience routine fluctuation.

Next, we have the “earnings” in the price-to-earnings ratio. Both trailing and forward P/E ratios have their limitations. Trailing P/E can feel like the more reliable of the two numbers because it’s based on facts. We take their actual earnings over the prior 12 months into account. But, in many situations, a company’s prior 12 months may have little to do with their next 12 months. As mentioned earlier, a company may have spent heavy the prior 12 months in preparation to ramp up the next 12 months. The trailing P/E won’t show us any of that. The forward P/E, on the other hand, is based on predictions. And predictions are quite educated guesses, but at the end of the day predictions are still guesses. A company may fall short of their predicted earnings or blow completely past them.

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