Compare your ACT score to the projected equivalent SAT score using a concordance chart. Your ACT Composite score (verbal and math) is comparable to the SAT Critical Reading and Math combined scores. Your ACT English/Writing score is comparable to the SAT Writing score.
Identify the percentile of your projected equivalent SAT scores.
Locate a definitive table of GRE percentiles and the test scores they encompass. Using the percentile you calculated from your projected SAT score based on ACT performance, you can identify the range of scores you can expect you receive on the GRE.
Remember that the GRE is a very different test from both the ACT and the SAT. The GRE assumes a body of knowledge that you acquired during college, whereas the ACT and SAT test skills and knowledge you acquired in high school. Preparing for the GRE is essential, as the question types are very different and can take some getting used to.