...Private Sub CommandButton7_Click()
Dim Wert As Double, Wert1 As Double
If Me.TextBox17.Value = "" Then
Me.TextBox17.Value = 0
End If
Wert1 = Replace(Me.TextBox17.Value, ",", ".")
Wert = Wert1 * 25.4
Me.TextBox18.Value = Replace(Wert, ",", ".")
Me.TextBox17.Value = Replace(Wert1, ",", ".")
End Sub