MedCalX supports a wide range of input units, you can select them from the keypad when an input field is focused. Your last unit choice of a particular input type (e.g. "sodium" or "body height") is remembered and automatically pre-selected the next time such an input field appears. This means that a global SI/US unit switch in not necessary and MedCalX does not have such a setting.

The active unit is displayed in the field, to the right of the value.

Change the unit from the keypad, with the buttons to the right of the numbers.

Tapping the active unit button will show you more information about the unit.

medcalx.ch/support.html#units