Staples Inc reported a better-than-expected quarterly profit on currency benefits and a slight uptick in sales to its business customers in North America, prompting the largest U.S. office supply chain to raise its profit outlook.

The news boosted Staples' shares by 8.3 percent to $15.40 in premarket trading.

Smaller rivals have also shown some improvement on the profit front lately, with Office Depot reporting a smaller-than-expected quarterly loss and OfficeMax posting better-than-expected earnings.

Many investors look at office supply retailers as a good gauge of the economy as demand for their products is closely tied to white-collar employment rates. Sales at all three chains have suffered in the weak U.S. economy, and the companies continue to keep a tight rein on costs to offset weak demand.

For the full year, Staples continues to see sales rising at a low-single-digit percentage rate.

Its net income rose to $176.4 million, or 25 cents a share, in the second quarter, from $129.8 million, or 18 cents a share, a year earlier.

Excluding a tax refund, it earned 22 cents a share. On that basis, analysts on average were expecting 19 cents a share, according to Thomson Reuters I/B/E/S.

Sales rose 5.2 percent to $5.82 billion, while analysts expected $5.65 billion.

For the full year, Staples now sees net earnings of $1.42 to $1.48 a share. In May the company cut its forecast to a range of $1.35 to $1.45 a share.

Excluding the tax refund in the second quarter, Staples sees full-year earnings of $1.39 to $1.45 a share.

(Reporting by Dhanya Skariachan; editing by John Wallace)