When a Splitter contains both RadPanes with size in percent and free-sized panes, the width of the fomer is calculated properly only after the initial loading and remains the same the the page is resized. Steps to reproduce: 1. <telerik:RadSplitter ID="RadSplitter1" runat="server" Width="100%"> <telerik:RadPane ID="RadPane1" runat="server" Width="10%"></telerik:RadPane> <telerik:RadSplitBar ID="RadSplitbar1" runat="server" ></telerik:RadSplitBar> <telerik:RadPane ID="RadPane2" runat="server"></telerik:RadPane> </telerik:RadSplitter> 2. Resize the browser. Result: the size of RadPane1 is not recalculated Temporary workaround: Set size in perncent to all RadPanes in the Splitter.