The way you ask the question does not yield the result you say you want.
The most direct solution does not involve choice lists. Define the field you want to fill as either real or integer. Either way, select the option "Unit of measure". Turn it on, add the calculator, select the category, and then the unit of measure.
Then, the user can enter any measure of that category, and with the calculator, the user can convert any measure's value to any other. You don't need lists.