When selecting a calculator for calculus, trigonometry, physics, and chemistry, the TI-89 is a strong option due to its capabilities, but it is prohibited on the ACT. The TI-84 Plus is a more accessible choice, though it lacks a computer algebra system (CAS), limiting its effectiveness for calculus. Users noted that while the TI-83+ can assist with integrals, it only provides numerical answers, which may not suffice for all calculus tasks. The TI-Nspire CAS is suggested as a potential future option, but it also may not be allowed on the ACT. For the TI-84 Plus, users need a linking cable and TI Connect software for file transfers. Recommendations for apps that assist with calculus, trigonometry, physics, and chemistry can be found online, and specific suggestions are sought by users. Overall, understanding the material is emphasized as crucial for effective calculator use.