Calculator application

I want to build a one page finance calculator in Rails.
The form has several fields on it. When I press the calculate button
it will return to the same page with the answer. If there is an error
it will also return to the same page with the appropriate error
messages.

I want it to be able to

  1. Carry over all the values on the form fields
  2. Use the rails validation methods

How can I acheive this without Active Record being tied to a particular
database table?