You will not be allowed to take a calculator in the exam hall. But you will get an onscreen calculator in CAT. It is advised to practice a little on this calculator so that you don’t do calculation mistakes in the exam. You will get the similar calculator in mock tests also. So while writing mocks, use the on-screen calculator and not your personal calculator.
The questions asked in CAT do not have tough or big calculations. If you have practiced enough, you can solve them without a calculator and it will save your time too. Use calculator only when you see tedious calculations.