How to do a calculation as the user fills up the fields?

Hi guys,

We have the following fields NumField1, Numfield2 and Numfield3. When a user fills NumField1 and Numfield2, NumField3 should display the quotient of the 2. So that will be NumField3 = Numfield1/Numfield2. How can we do this calculation on the custom NewForm of the list.

Thanks,

Arnel