Markup to reproduce the issue: <telerik:RadEditor ID="RadEditor1" runat="server" EditModes="Design" RenderMode="Lightweight"> <Tools> <telerik:EditorToolGroup> <telerik:EditorTool Name="Bold" /> <telerik:EditorTool Name="Italic" /> <telerik:EditorTool Name="Underline" /> </telerik:EditorToolGroup> </Tools> <Content> <p>test</p> <p>test</p> </Content> </telerik:RadEditor> Resolution: <script type="text/javascript"> (function ($UI) { $UI.SizerLightweight.prototype._calcUiHeight = function () { var that = this, view = that._view, editor = that.editor, calculator = new $UI.DimensionsCalculator(editor), toolBarContainer = view.toolBarContainer(), toolBarHeight = that._getComponentHeight(toolBarContainer), paddings = calculator.getComputedSizes(editor.get_element(), ["padding-top", "padding-bottom"]), margins = calculator.getComputedSizes(toolBarContainer, ["margin-bottom"]), borders = calculator.getComputedSizes(view.contentAreaContainer(), ["border-top-width", "border-bottom-width"]), modesHeight = that._getComponentHeight(view.modesRowContainer()), modulesHeight = that._getComponentHeight(view.modulesContainer()); return toolBarHeight + modesHeight + modulesHeight + margins + borders + paddings; } })(Telerik.Web.UI.Editor.UI); </script>