How to calculate the sum of the datatable column in asp.net?

To calculate the sum of a column in a DataTable use the DataTable.Compute method.

Example of usage from the linked MSDN article:

DataTable table = dataSet.Tables["YourTableName"];

// Declare an object variable.
object sumObject;
sumObject = table.Compute("Sum(Amount)", string.Empty);

Display the result in your Total Amount Label like so:

lblTotalAmount.Text = sumObject.ToString();