The economy expanded in the third quarter at the fastest rate in almost two years as Americans stepped up spending on services such as health care and companies invested more in software.

Gross domestic product climbed at a revised 4.1 percent annualized rate, the strongest since the final three months of 2011 and up from a previous estimate of 3.6 percent, Commerce Department data showed today inWashington. The gain exceeded the most optimistic projection in a Bloomberg survey.

Inventories accounted for a third of the increase in GDP in the third quarter, showing companies were confident about the prospects for demand. Stronger retail sales in October and November underscore the Federal Reserve’s view that the world’s largest economy is improving.