We'd all like to invest as successfully as the legendary Warren Buffett. He calculates return on invested capital (ROIC) to help determine whether a company has an economic moat -- the ability to earn returns on its money beyond that money's cost.

ROIC is perhaps the most important metric in value investing. By determining a company's ROIC, you can see how well it's using the cash you entrust to it, and whether it's actually creating value for you. Simply put, ROIC divides a company's operating profit by the amount of investment it took to get that profit:

ROIC = Net operating profit after taxes / Invested capital

This one-size-fits-all calculation cuts out many of the legal accounting tricks (such as excessive debt) that managers use to boost earnings numbers, and provides you with an apples-to-apples way to evaluate businesses, even across industries. The higher the ROIC, the more efficiently the company uses capital.

Ultimately, we're looking for companies that can invest their money at rates that are higher than the cost of capital, which for most businesses lands between 8% and 12%. Ideally, we want to see ROIC greater than 12%, at minimum. We're also seeking a history of increasing returns, or at least steady returns, which indicate that the company's moat can withstand competitors' assaults.

Let's look at Amazon.com (Nasdaq: AMZN) and two of its industry peers to see how efficiently they use capital. Here are the ROIC figures for each company over several time periods:

Company

TTM

1 year ago

3 years ago

5 years ago

Amazon.com

54.1%

68.4%

82.7%

255.1%*

Barnes & Noble (NYSE: BKS)

0%

7.1%

9.9%

12.3%

eBay (Nasdaq: EBAY)

15.9%

15.1%

15.2%

17.3%

Source: Capital IQ, a division of Standard & Poor's. TTM = trailing 12 months. *Assumes 22.2% tax rate, which the company used in the subsequent filing.

While Amazon.com has consistently given us high returns on invested capital, they have steadily declined over the past three years. But such ridiculous returns were never going to last, and current margins are still plenty high. Barnes & Noble offers us less appealing ROIC, and also displays steadily declining returns, a very troubling situation that reinforces the competitive threats for the book retailer. eBay has reliably maintained returns on capital within the 15% range over the past three years – an acceptably high level for this somewhat defensible business.

Businesses with consistently high ROIC are efficiently using capital. They can use their extra returns to buy back shares, further invest in their future success, or pay dividends to shareholders. (Warren Buffett especially likes that last part.)

To unearth more successful investments, dig a little deeper than the earnings headlines, and check up on your companies' ROIC.

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Jim Royal, Ph.D., does not own shares in any company mentioned. Amazon.com and eBay are Motley Fool Stock Advisor recommendations. Motley Fool Options has recommended a bull call spread position on eBay. Try any of our Foolish newsletter services free for 30 days. The Motley Fool has a disclosure policy.