Calculating a field in an adobe form with a constant number

Hello.  I am new to creating forms and am trying to multiply a number by a constant.  In my example, I have a field called MonthlyRent.  This field is entered as currency.  I simply want to multiply that MonthlyRent by 12 to show the AnnualRent.  How do I go about doing this in Adobe?  I have about 25 different fields to calculate.  Thank you!