What are the differences between SAT and ACT?

Timing 

The ACT is more time-bound having more questions in a similar timeframe as the SAT

Math

The ACT covers more math topics with a larger focus on Geometry and Trigonometry.  SAT, however, focuses more in-depth on Algebra.

Science

Although the SAT incorporates science questions throughout the exam, it does not have an independent science section. ACT has a whole section dedicated to science only.

English

There is not much difference in this section as both the ACT and SAT have this section that assesses grammar skills.

Reading

SAT Reading has evidence-support reading questions that test reasoning ability and is considered to have a higher language level and more difficult reading passages than ACT. 

Writing

Only the ACT offers an optional ACT essay writing portion.