diff --git a/web/skins/classic/views/js/monitor.js b/web/skins/classic/views/js/monitor.js
index aa595a31f..bcc4681bb 100644
--- a/web/skins/classic/views/js/monitor.js
+++ b/web/skins/classic/views/js/monitor.js
@@ -48,6 +48,7 @@ function updateMonitorDimensions(element) {
       form.elements['newMonitor[Height]'].value = dimensions[1];
     }
   }
+  update_estimated_ram_use();
   return false;
 }
 
@@ -139,6 +140,9 @@ function initPage() {
       form.submit();
     };
   });
+  document.querySelectorAll('input[name="newMonitor[ImageBufferCount]"],input[name="newMonitor[Width]"],input[name="newMonitor[Height]"]').forEach(function(el) {
+    el.oninput = window['update_estimated_ram_use'].bind(el);
+  });
 
   document.querySelectorAll('select[name="newMonitor[Function]"]').forEach(function(el) {
     el.onchange = function() {
@@ -306,6 +310,15 @@ function random_WebColour() {
   );
 }
 
+function update_estimated_ram_use() {
+  var buffer_count = document.querySelectorAll('input[name="newMonitor[ImageBufferCount]"]')[0].value;
+  var width = document.querySelectorAll('input[name="newMonitor[Width]"]')[0].value;
+  var height = document.querySelectorAll('input[name="newMonitor[Height]"]')[0].value;
+  var colours = document.querySelectorAll('select[name="newMonitor[Colours]"]')[0].value;
+
+  document.getElementById('estimated_ram_use').innerHTML = human_filesize(buffer_count * width * height * colours, 0);
+}
+
 function updateLatitudeAndLongitude(latitude, longitude) {
   var form = document.getElementById('contentForm');
   form.elements['newMonitor[Latitude]'].value = latitude;