Function SumStringChars(ByVal strStringToSum As String) As Integer Dim chrCharToSum As Char Dim i As Integer Dim intResult As Integer = 0 Try For i = 0 To strStringToSum.Length - 1 chrCharToSum = strStringToSum.Chars(i) If Not Char.IsDigit(chrCharToSum) Then Throw New ArgumentException End If intResult += Val(chrCharToSum) Next Return intResult Catch ex As Exception Throw End Try End Function Private Sub TextBox1_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox1.TextChanged TextBox2.Text = SumStringChars(TextBox1.Text) End SubHope that helps,