InfoDrainage allows you to easily change the units when creating a model. From the  Preferences dialog, click the Edit button to launch the Units dialog.

Under the Region drop-down menu, a range of countries are available, this will set the Units most likely used in these Countries. If you choose User Defined, you can change the units for each measurement. The Units are remembered when you next open a New design as well as within each design. If you wish to re-use a Units template, you can use the Save As option to save an .idux file. Using the Open icon you can reload an existing Units file.

The software includes both metric and US/Imperial units by default. Click the units for each measurement to display a drop-down menu, and select the appropriate units for your project or region. 

Alternatively, you can create custom Unit Translation sets to quickly change the units used by InfoDrainage.  Read more about Units Translation.