As you've noticed, you can switch the units on the dimensions displayed on the screen in the Dimensions dialog box. You can select either inches or millimeters.

However, when it comes to inputting dimensions, you can currently only input dimensions in millimeters. The only exception is entering body dimensions in the fit advisor. Those dimensions can be entered in either millimeters or inches.

For all other dimensions in BikeCAD, to save yourself a bit of hassle, be aware that you can enter dimensions in the form of mathematical expressions. If you want to input 18.5", rather than converting that on your own to 469.9mm and entering that, you could simply type in: 18.5*25.4 .