title: VBA excel calculating in userform
body: <p>i have a userform that includes combox and two textboxes
  the user choose a chemical (text) from the combobox1 and enters the height of the chemical in textbox1 (number).
  then according to the chemical he choose i define the density and area of the tank of the chemical.
  then i need to calculate: textbox2.value=density*area*textbox1.value
  density and area are different for every chemical.
  the equation is mass=density*area*volume.
  i tried this code:
      Private Sub ComboBox1_Change()
      Dim chem As String</p>
  
  <pre><code>chem = ComboBox1.Value
      mychem
  
  End Sub
  Sub mychem()
  Dim density As Double
  Dim volume As Double
  
  If chem = "Sodium" Then
  area = 22
  density = 1.058
  End If
  If chem = "HCl 9%" Then
  area = 22
  density = 1.043
  End If
  If chem = "alum" Then
  area = 70
  density = 1.163
  End If
  If IsNumeric(Txtheight.Text) Then
                  txtmass.Value = density *area * CDbl   (Txtheight.Value)
  
                     End If
  end sub
  </code></pre>