The GridBoundColumn object has an Aggregate property that you can set to specify a function for aggregating the values that the column displays. The Aggregate property can be set to any of the following values: "Sum", "Min", "Max", "Last", "First", "Count", "Avg", or "Custom". When you set the Aggregate property to "Custom", the grid raises an OnCustomAggregate event, where you can calculate the aggregate in server-side code and assign the result to the Result property of the event arguments object.
<telerik:GridBoundColumn Aggregate="Count" DataField="CustomerID" DataType="System.String" HeaderText="CustomerID" SortExpression="CustomerID" UniqueName="CustomerID"> </telerik:GridBoundColumn>
The grid calculates aggregated values if the ShowFooter property is set to True. The grid footer displays aggregates that are calculated based on all the data from the data source, except for any values that are excluded by a filter expression.
When grouping is enabled in the column, you can display group aggregates by setting the ShowGroupFooter property to True: