When you're dealing with a company that has no earnings, the price-to-sales ratio (P/S) can be a handy alternative to a price-to-earnings (P/E) ratio. If there are no earnings, you can't calculate a P/E, but as long as a company has sales, or revenues, you can calculate a P/S. (If company has no revenues, why the heck are you looking at it?)

The P/S divides the market capitalization of a company by its past 12 months' revenues. Remember that the market cap is the market's current value for the company, determined by multiplying the current share price by the number of shares outstanding.

Take a look at up-and-comer XM Satellite Radio (NASDAQ:XMSR). XM has 268.2 million shares outstanding, priced at \$13.67 a share (as of market-close on Sept. 15, 2006), so its market capitalization is \$3.67 billion. The company had \$766.1 million in sales over the past four quarters (as of June 30, 2006), so its P/S would be 4.79 -- \$3.67 billion divided by \$766.1 million.

It can be useful to compare the P/S with sales growth. A high P/S isn't necessarily bad if sales are growing rapidly. XM's compound annual revenue growth has been a whopping 155.5% over the past three years.

The P/S is especially handy with start-ups, small-cap companies, and unprofitable firms; you can use it to determine how a company is faring relative to its competitors. XM is still unprofitable, but it has a P/S of 4.79. Compare this with competitor Sirius Satellite Radio (NASDAQ:SIRI); it has a P/S of almost 13.6, but its compound annual sales growth rate has reached 359.5% over three years.

In addition, there are always years during recessions when companies in a certain industry, such as the auto industry, are unprofitable. This doesn't mean they're all worthless and lack an easy means of comparison. You can simply apply measures such as the P/S instead of the P/E -- sizing up how much you're paying for a dollar of sales instead of a dollar of earnings.

Despite its usefulness, though, the P/S should never be the only number you crunch. The P/S can sometimes give you a nice context for a company's value relative to its industry peers, but while sales growth is great, revenues must be transformed into meaningful and rising earnings to make shareholders happy.

Some companies have massive and growing revenues but few earnings to show for it. The amount a company earns from its sales will eventually drive the value of the business and the stock over the long term, along with its cash position and free cash flow. Learn about other ways for Foolish investors to measure value by reading more about valuation.

XM Satellite Radio is a Motley Fool Rule Breakers recommendation. Join David Gardner and his team in their search for the next ultimate growth stock with a free 30-day trial .

Shruti Basavaraj, Adrian Rush, and Katrina Chan contributed to this article, which was originally written by Selena Maranjian. None of them owns shares of any company mentioned above. The Motley Fool has a fundamental disclosure policy .