simply add the following code
grid.Column(header: "No."
,format: item => item.WebGrid.Rows.IndexOf(item) + 1
+ Math.Round(Convert.ToDouble(grid.TotalRowCount / grid.PageCount) / grid.RowsPerPage)
* grid.RowsPerPage * grid.PageIndex)
Check this link for more info
hope this will be helpful to someone