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 Staples (NYSE: SPLS) 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

Staples

11.7%

11.2%

19.3%

20.6%

Office Depot (NYSE: ODP)

0.8%*

(2.9%)*

14.7%

13%

OfficeMax (NYSE: OMX)

2.8%**

2.8%**

4.2%

1.8%

Source: Capital IQ, a division of Standard & Poor's. TTM = trailing 12 months. *Uses 2006's effective tax rate of 28.8%. **Uses 2007's effective tax rate of 37.1%.

Clearly, this is an industry that has been hit hard by the economic downturn. Staples offers us returns on invested capital that are slightly below our desired 12%, but more distressingly, that have declined substantially from five years ago. Still, Staples is a cut above its peers, which have suffered much more recently. The low returns of Office Depot and OfficeMax cannot persist indefinitely without investors taking some action, but I'd expect the performance of the strongest player -- Staples -- to improve as overall conditions strengthen.

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. Staples is a Motley Fool Stock Advisor recommendation. Try any of our Foolish newsletter services free for 30 days. True to its name, The Motley Fool is made up of a motley assortment of writers and analysts, each with a unique perspective; sometimes we agree, sometimes we disagree, but we all believe in the power of learning from each other through our Foolish community. The Motley Fool has a disclosure policy.