<input type="number" style="text-align: center;" name="weight">
<div id="text"></div>

var d6;
var bk4=39.373127;
var bk5=66.118203;
var bk6=0.14327702;
var bk7;
var bk8;
var bk11;
var ae11;
var af19;

$(document).ready(function(){
$(function() {

    $('input[name="weight"]').change(function() {

    d6=($('select[name="dose"]').val())/($('input[name="weight"]').val());
    });
    $('input[name="weight"]').change(function() {
     bk8= (((d6/50)*(39.373127/0.14327702))+((d6/50)*(66.118203/bk7)));
    });
    document.getElementById("text").innerHTML=bk8;
    });