How do I create a function where the user chooses input and output units?

I need to create a function in which the user is prompted to choose whether they would like to input a value in 'in' or 'cm' or 'm' let's say. But I also need them to be able to choose whether they want the output in 'in' or 'cm' or 'm'.

And this is all for a function calculating the volume of a Sphere. How would one go about doing this code wise?