function doCalculate() {
  var length = document.getElementById("length").value * document.getElementById("length_multiplier").value;
  var width = document.getElementById("width").value * document.getElementById("width_multiplier").value;
  var depth = document.getElementById("depth").value * document.getElementById("depth_multiplier").value;
  
  var tons = length*width*depth*1.5;
  
  tons = Math.round(tons*Math.pow(10,2))/Math.pow(10,2)
  
  document.getElementById("output").innerHTML = "You need " + tons + " tons of material.";
}