function calcROI(){
  //reset our CPI and ROI
  document.getElementById('cpi_value').innerHTML = '0.00';
  document.getElementById('roi_value').innerHTML = '0';
  document.getElementById('roi_profits').innerHTML = '0';
  
  var var1 = document.roi.cost.value;
  var var2 = document.roi.recipients.value;
  var var3 = document.roi.daily.value;
  var var4 = document.roi.perweek.value;
  var var5 = document.roi.weeks.value;
  var var6 = document.roi.percentRecipients.value / 100;
  var var7 = document.roi.salesOrder.value;
  var var8 = document.roi.advertising_increase.value;
  var var9 = document.roi.increase.value / 100;
  var var10 = document.roi.product_increase.value;
  
  //requirements
  if(!(var1 > 0)){
    alert("Product cost must be greater than $0");
    return;
  }
  
  //calculate
  var8 = var2 * var6 * var7;
  var10 = var8 * var9;  

  var precision=4;
  //show
  document.getElementById('cpi_value').innerHTML = (var1 / (var2 * var3 * var4 * var5)).toFixed(precision);
  document.getElementById('roi_value').innerHTML = var2 * var6 * var7 / var1 * 100;
  document.getElementById('roi_profits').innerHTML = var2 * var6 * var7 * var9 / var1 * 100;
  document.roi.product_increase.value = var10;
  document.roi.advertising_increase.value = var8;
}