The price-to-earnings ratio (P/E ratio) is the ratio for valuing a company that measures its current share price relative to its per-share earnings (EPS).

In general, a high P/E suggests that investors are expecting higher earnings growth in the future compared to companies with a lower P/E. A low P/E can indicate either that a company may currently be undervalued or that the company is doing exceptionally well relative to its past trends. When a company has no earnings or is posting losses, in both cases P/E will be expressed as “N/A.” Though it is possible to calculate a negative P/E, this is not the common convention.

PEG ratio

While a low P/E ratio may make a stock look like a good buy, factoring in the company's growth rate to get the stock's PEG ratio may tell a different story. The lower the PEG ratio, the more the stock may be undervalued given its future earnings expectations. Adding a company's expected growth into the ratio helps to adjust the result for companies that may have a high growth rate and a high P/E ratio.

The degree to which a PEG ratio result indicates an over or under-priced stock varies by industry and by company type. As a broad rule of thumb, some investors feel that a PEG ratio below one is desirable.

According to well-known investor Peter Lynch, a company's P/E and expected growth should be equal, which denotes a fairly valued company and supports a PEG ratio of 1.0. When a company's PEG exceeds 1.0, it's considered overvalued while a stock with a PEG of less than 1.0 is considered undervalued.