Stock index futures added to gains on Friday after government data showed retail sales in July posted their biggest gain since March.

S&P 500 futures rose 9.5 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 added 86 points and Nasdaq 100 futures rose 12.75 points.

(Reporting by Chuck Mikolajczak)