Stock index futures added to gains on Friday as data on September retail sales came in stronger than expected.

Futures had already been in positive territory, shrugging off a credit rating downgrade of Spain, propelled by strong earnings from Google Inc and hopes the euro zone will continue progress toward a solution to its debt crisis.

S&P 500 futures rose 15.3 points and were above fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures gained 122 points, and Nasdaq 100 futures rose 25.5 points.

(Reporting by Ryan Vlastelica; Editing by Padraic Cassidy)