Format excel column to decimal doing export from c#

Here you go, one complete method. Just send a DataTable and a file name and this does the rest. This snippet will also make the header row gray with bold text and will auto fit the columns.

using OfficeOpenXml;
using OfficeOpenXml.Style;

public void ExportToExcel(DataTable dt, string FileName)
{
    //create a new byte array       
    byte[] bin;

    //create a new excel document
    using (ExcelPackage excelPackage = new ExcelPackage())
    {
        //create a new worksheet
        ExcelWorksheet ws = excelPackage.Workbook.Worksheets.Add(FileName);

        //add the contents of the datatable to the excel file
        ws.Cells["A1"].LoadFromDataTable(dt, true);

        //auto fix the columns
        ws.Cells[ws.Dimension.Address].AutoFitColumns();

        //loop all the columns
        for (int col = 1; col <= ws.Dimension.End.Column; col++)
        {
            //make all columns just a bit wider, it would sometimes not fit
            ws.Column(col).Width = ws.Column(col).Width + 1;

            var cell = ws.Cells[1, col];

            //make the text bold
            cell.Style.Font.Bold = true;

            //make the background of the cell gray
            var fill = cell.Style.Fill;
            fill.PatternType = ExcelFillStyle.Solid;
            fill.BackgroundColor.SetColor(ColorTranslator.FromHtml("#BFBFBF"));

            //make the header text upper case
            cell.Value = ((string)cell.Value).ToUpper();
        }

        //convert the excel package to a byte array
        bin = excelPackage.GetAsByteArray();
    }

    //clear the buffer stream
    Response.ClearHeaders();
    Response.Clear();
    Response.Buffer = true;

    //set the correct contenttype
    Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";

    //set the correct length of the data being send
    Response.AddHeader("content-length", bin.Length.ToString());

    //set the filename for the excel package
    Response.AddHeader("content-disposition", "attachment; filename=\"" + FileName + ".xlsx\"");

    //send the byte array to the browser
    Response.OutputStream.Write(bin, 0, bin.Length);

    //cleanup
    Response.Flush();
    HttpContext.Current.ApplicationInstance.CompleteRequest();
}