How much science do I need to know to get a good ACT science score?

Absolutely none. The ACT “science” section has absolutely nothing to do with physics, chemistry, or biology. It should really be called the ACT “comparing graphs and tables and using common sense” section. I teach this section for a living, and I’m one of the worst scientists in the world. For instance, a question might ask, “during which year were mercury levels in X lake the highest?” Then they’ll show you a graph of fish deaths over multiple years in the lake, and explain that mercury leads to dead fish. Put two and two together and you have your answer. If you’re not a strong science student, don’t worry – the ACT requires zero actual science knowledge to master – just proper strategy.