Do Loops

This:

Length = UCase(Left(code, 1, 1))

should only be:

Length = UCase(Left(code, 1))

Dom

Length and Diameter should be declared as String's by the look of it.

Dom

Still needs a bit of work but hopefully this will point you in the right direction:

Sub DetailProduct()

Dim code As String
Dim Length As String
Dim Diameter As String
Dim Colour As String
Dim ok As Boolean

ok = False

Do
    code = InputBox("Please enter the 3 character product code.")
    
    If code = "" Or code = "END" Then
        Exit Sub
    End If

    If IsNumeric(code) Then
        ok = False
        MsgBox "Error - you have entered the numeric input."
    Else
        If Len(code) <> 3 Then
            ok = False
            MsgBox "error- please enter 3 character product code."
        Else
            ok = True
           Length = UCase(Left(code, 1))
  
            Diameter = UCase(Mid(code, 2, 1))
     
            Colour = UCase(Right(code, 1))
        
            Select Case Length
                Case "A"
                    Length = 250
                Case "B"
                    Length = 500
                Case "C"
                    Length = 1000
                Case "D"
                    Length = 3000
                Case Else
                    MsgBox "error"
            End Select
        
            Select Case Diameter
                Case "K"
                    Diameter = 30
                Case "L"
                    Diameter = 45
                Case "M"
                    Diameter = 60
                Case "N"
                    Diameter = 100
                Case Else
                    MsgBox "error"
            End Select
        
            Select Case Colour
                Case "W"
                    Colour = "red"
                Case "X"
                    Colour = "green"
                Case "Y"
                    Colour = "blue"
                Case "Z"
                    Colour = "white"
                Case Else
                    MsgBox "error"
            End Select
        End If
    End If

MsgBox "The input " & code & " would produce the cylinder is " & Length & "mm by " & Diameter & "mm and is " & Colour

Loop

End Sub

Dom