FormsCentral doesn't support calculation on the online fillable form. You can do this with a PDF form if you have Acrobat... you would need to create your form outside of FormsCentral and add your calculations to the PDF in Acrobat then import the PDF in FormsCentral to collect data. Note that PDF form don't support PayPal payments.
Thank you for the quick response!
I do have Acrobat, once I create the form in acrobat, there isn't a way to convert it to a web form, correct?
As a followup, the forms I've created have been made in Word and then imported into Acrobat. Is there an easy way to edit the form after it's created, and not have to edit the Word document and re import it everytime?
This is correct, PDF form can't be converted into a web form.
There are some touch-up tools in Acrobat so that you can fix typos and some text but for major changes you need to go back to your word doc and export to PDF with your new changes. You might find more information on the Acrobat forum regarding this : http://forums.adobe.com/community/acrobat