If you have a text box that contain 16 OZ, you can use the code

SDK as String
SDK = Right(txtAmount.text, 2)
' SDK will = OZ

This code will give you the last 2 caracters from the right. You can also write

SDK = Left(txtAmount.text, 2)
'SDK will = 16

To get the 2 first caracters from the left and

SDK = Mid(txtAmount.text, 2, 4)
SDK will = ' OZ"

The command MID will put the 4 caracters from the second caracters on in of your txtAmount.text in the variable SDK.