show total price

show total price

hi there
I have 3 text boxes, one for show price, other for quantity & the last one is for Total price. I load these data from the database, the default quantity is always one. but the user can change it. 

what this code does is, it loads the price from the database, and shows the quantity "one" but does not show the total price, I have to change the price manually & only then it shows the price. 

please help me

here is my code. 
  1. $('#deviceunitPrice1, #devicequantity1').change(function(){
var rate = parseFloat($('#deviceunitPrice1').val()) || 0;
var box = parseFloat($('#devicequantity1').val()) || 0;

$('#devicetotalprice1').val(rate * box);    
});