function calc(form)
{

  if(isNaN(form.radius.value))
  {
     form.radius.value = 0;
  }

  if(form.radius.value.length > 32)
  {
     form.radius.value = 0;
  }

  var radius = parseFloat(form.radius.value);

  if(isNaN(radius))
  {
     radius = 0;
  }

  var area = Math.PI * Math.pow(radius,2) / 43560.0;
  document.getElementById("cirarea").innerHTML = chopTo2(area);

}

function chopTo2(raw)
{
  strRaw = raw.toString();
  if(strRaw.length - strRaw.indexOf(".") > 2)
    strRaw = strRaw.substring(0,strRaw.indexOf(".") + 3);
  return strRaw;
}