CHICAGO - Wal-Mart Stores Inc will roll out shelf-scanning robots in approximately 40 stores to replenish inventory faster on its shelves and save store employees time when products run out.

The company has been piloting such robots in a handful of stores.

"If you are running up and down the aisle, and you want to decide if we are out of Cheerios or not, a human doesn't do that job very well and they don't like it," Chief Technology Officer Jeremy King told Reuters.

The robots are about 2 feet in size and come with a tower on their backs that is fitted with cameras, which scan aisles to check stocks, missing items and if products have been left in the wrong place by customers.

They are 50 percent more productive, can scan shelves three times faster than their human counterparts and significantly improve accuracy levels, King said. Store employees are only able to scan shelves about twice a week.

Out-of-stocks are a big challenge for retailers, which miss out on sales every time a shopper is unable to find a product on store shelves.