Update field on form after values of other input fields have changed

Hi VC guru's

I have a what I thought very simple requirement.

I have a form where the end user can put in two variables.

A third field contains a formula based on these two variables.

I would like this field to update automatically either (preferabely) each time one of the input field changes or alternatively when I hit the submit button.

It does not happen automatically ;(

I thought maybe I need to declare an action but couldn't work out how that would work.

Many thanks in advance,

Jan