Under System, select the unit system to use. Unit systems are sets of quantity types for mass, length, time, and so on.

The options under System include SI, CGS, English Engineering, British Technical, US Customary, US Engineering, or Custom. Only Custom enables you to redefine a quantity type (for example, to use inches for the dimensions in a file that otherwise used SI units).

The most common quantity types appear in the main Options dialog box; to see all quantity types, click More Units.

Select or clear Always convert units to Preferred Units.

If Always convert units to Preferred Units is selected, the units of entered quantities are immediately converted to those set in this dialog box.

For example, if you have set Velocity to [m s^-1] in this dialog box to make that the preferred velocity unit, and elsewhere you enter 20 [mile hr^-1] for a velocity quantity, the entered value is immediately converted and displayed as 8.94078 [m s^-1].