Basic JavaScript addition problem

Question

I'm trying to create a customized calculator in an APEX popup page using JavaScript and I keep getting caught on JavaScripts use of the + for both addition and concatenation operators. Is there a way to fix this? I'm ONLY using basic arithmetic operations, but I can't seem to find a way to declare a variable as a NUMBER the way I can in PL/SQL.
Here's my function:
<script language="JavaScript" type="text/javascript">
  function calcBMI(formItem1, formItem2, formItem3, formItem4) {
    var bmift = document.getElementById(formItem1).value;
    var bmiin = document.getElementById(formItem2).value;
    var bmiht = bmift*12;
    var bmiht = bmiht + bmiin;
    var bmiwt = document.getElementById(formItem3).value*703;
    var bmi   = bmiwt/bmiht^2;
    document.getElementById(formItem4).value = bmi;
</script>I need to add the height in feet to the height in inches, but instead of 5'4" = 64, I'm getting 604.

Answer 1

So here's the final function:
<script language="JavaScript" type="text/javascript">
  function calcBMI(formItem1, formItem2, formItem3, formItem4) {
    var bmift = document.getElementById(formItem1).value;
    var bmiin = document.getElementById(formItem2).value;
    var bmiht = bmift*12 + parseInt(bmiin);
    var bmiwt = document.getElementById(formItem3).value*703;
    var bmi   = Math.round(bmiwt/(bmiht*bmiht)*100)/100;
    document.getElementById(formItem4).value = bmi;
</script>