A calculator can definitely become a crutch if you use it for every small problem. When that happens, it gets harder to do basic operations quickly in your head, and that can hurt you on the ASVAB, especially in Arithmetic Reasoning and Mathematics Knowledge. The good news is that mental math is a skill you can rebuild with steady practice.
Before you do anything else, decide how much time you have before your test. Your study plan should look different if your ASVAB is only a few days away versus a few months away.
The most important thing you can do is take practice tests. They will help you identify your strengths and weaknesses. They also help you get used to solving problems under time pressure, which is a big part of doing well on the ASVAB.
The next best step, especially if your test is coming up soon, is to work lots of math problems without a calculator. Start at about a sixth-grade level and build from there. You can use a textbook, a workbook, or search online for basic math drills involving fractions, decimals, percentages, and order of operations. When you are comfortable with these, move into the basics of algebra and geometry. Memorize formulas for area, volume, etc.
As you practice, do not rush to check the answer the moment you get stuck. Try to work through it on your own. If you miss a problem, stop and figure out why. Was it a careless mistake? Did you forget a math rule? Did you misunderstand the question? That reflection is where a lot of improvement happens.
To strengthen your mental math skills, practice a little every day. Review multiplication facts, do short division by hand, estimate answers before solving, and practice breaking problems into smaller steps. For example, instead of seeing 48 × 5 as one big problem, think of it as 50 × 5 = 250, then subtract 10 to get 240. Small strategies like that make mental math faster and easier.
Lastly, be encouraged! Improvement is achievable. Fifteen minutes of daily calculator-free practice can make a big difference. The more you force your brain to do the work, the more confident and accurate you will be on test day.