The SAT is said to correlate about as well with IQ as two different IQ tests correlate with one another. In his book Real Education (pg 69-70), Charles Murray claims that only 17 year olds capable of getting an 1180+ on the post 1995 SAT (Critical Reading + Math) are true college material. Although 35% of American 17 year olds take the SAT, he estimates that only 10% of 17 year would score 1180+ if all of them took it.. In other words, 1180+ is equivalent to an IQ of 120+.
In his book Coming Apart (pg 375) he estimates an SAT score of 1400 is equivalent to an IQ of 135. From these two data points, we can create the following formula for converting SAT into IQ equivalents:
IQ = 39.545 + 0.068(SAT score)
Note, this formula only applies to the post 1995 SAT. Before 1995 the SAT was much harder.
This formula works well for individuals, but should never be used to estimate the median IQ of a college. This is because elite colleges select for SATs, so the estimated median IQ will be artificially inflated by a selection bias.