protected void Page_Load(object sender, EventArgs e) { if (Page.IsPostBack == false && GridWeb1.IsPostBack == false) { // Assign your own custom calculation engine to GridWeb GridWeb1.CustomCalculationEngine = new GridWebCustomCalculationEngine(); // Access the active worksheet and add your custom function in cell B3 GridWorksheet sheet = GridWeb1.ActiveSheet; GridCell cell = sheet.Cells["B3"]; cell.Formula = "=MYTESTFUNC(9.0)"; // Calculate the GridWeb formula GridWeb1.CalculateFormula(); } }