Writing Values To Database.

I'm a newb - so if this is a stupid question, my apologies.

I have a simple form that users fill out and then that info is written to a database. What I'd like to do is perform a calcution on a number and then write those numbers to other records in the database.

So for instance, if someone enters 12 in one field1, it would multiply that by 3 and that would be saved as field2, then mulitply it by 45 and that would be field3, then add field2 and field3 and save that as field4.

I was thiking of passing the form variables to a second page instead of writing them to the database, then using variables in the second page to perform the calculations and then have the second page write all the info to the database, but I thought there had to be a better solution.