Unit Conversion

Try putting this in the userform's code module

Private Sub UserForm_Initialize()
    With Me.ComboBox1
        .ColumnCount = 2
        .ColumnWidths = ";0"
        .AddItem "bar"
        .AddItem "psi"
        .AddItem "pascal"
        Rem conversion units from .listindex to bar
        .List(0, 1) = 1
        .List(1, 1) = 14.5
        .List(2, 1) = 100000#
        .ListIndex = 0
    End With
    TextBox1.Text = vbNullString
End Sub

Private Sub ComboBox1_Click()
    With ComboBox1
        TextBox1.Text = CStr(Val(TextBox1.Tag) * Val(.List(.ListIndex, 1)))
    End With
End Sub

Private Sub TextBox1_AfterUpdate()
    Dim Amount As Double
    Dim AmountInBars As Double
    Amount = Val(TextBox1.Text)
    With ComboBox1
        AmountInBars = Amount * .List(.ListIndex, 1)
    End With
    TextBox1.Tag = AmountInBars
End Sub