Quick Answer
On the standard unweighted scale, 4.0 is the highest possible GPA. On weighted scales that include AP or Honors courses, GPAs above 4.0 (sometimes up to 5.0) are possible.
Weighted vs Unweighted Maximums
An unweighted GPA caps at 4.0 regardless of course difficulty. A weighted GPA gives extra credit for AP, IB, or Honors classes, which is how some students report GPAs above 4.0. Colleges typically recalculate applicants' GPAs using their own standardized formula, so a high weighted GPA doesn't always translate directly.