I have created a calculator in Python using Tkinter  module,though I converted it to exe but I am not able to convert it to apk.please tell me how to do so?
I personally haven't seen anyone do that. I think it would be best to try and re-make you calculator in the Kivy framework if you want to later turn it into an APK using bulldozer.  Tkinter is decent for beginners but if you want to have nice Desktop UI's use PyQT5 and if you're interested in making mobile apps use Kivy. Tkinter is just a way to dip into using GUIs in python.
How to convert py file to apk?