function calcpayment() {
   var amt = document.frm.amt.value;
   var ir = document.frm.ir.value / 1200;
   var term = document.frm.term.value * 12;
   var total=1;

   for (i=0;i<term;i++) {
      total = total * (1 + ir);
   }
   mp = amt * ir / ( 1 - (1/total));

  // use Math object to chop all numbers after 2 digits
  document.frm.payment.value = Math.round(mp*100)/100;
}