No, calculators are not allowed in the IBPS PO exam, neither in the prelims nor in the mains.
The online exam interface provides an on-screen calculator only in sections where heavy calculation is expected.
Carrying a physical calculator, mobile phone or any electronic device into the exam hall is treated as unfair means.
Practice doing quick calculations mentally and on rough sheets, which are provided at the centre.
Read the admit card instructions on the exam day — prohibited items are listed there clearly.