In this post Calculate Sum in GridView Footer Row with Asp.Net

User Defined Method
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private void calculateSum()
    {
        Int64 grandtotal = 0;
        foreach (GridViewRow row in GridView1.Rows)
        {
          
          grandtotal=grandtotal+Convert.ToInt64(row.Cells[3].Text); //Where Cells is the column. Just changed the index of cells
        }
        GridView1.FooterRow.Cells[2].Text = "Grand Total";
        GridView1.FooterRow.Cells[3].Text = grandtotal.ToString();
    }

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GridView DataBound Event Coding
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calculateSum();

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