When I am designing in Tinkercad, I mainly design using millimeters (mm), because most of the time I am using a .04 mm nozzle on my 3D Printer. If I want to make a cover for something, like my Alexa Device, I will measure it in millimeters.

I made this ruler in Tinkercad so it would be easier to measure items in millimeters.

First, I can place the ruler on top of an object and see the object through the ruler.

Second, I can place an object on top of the ruler and against the left side and get a measurement.

I printed three rulers in different colors, because every time I changed the filament in the printer for another project, I used the ruler to make sure the printer was calibrated correctly. If I got a good print on the ruler, there was a high chance that the next print would come out good.

The ruler is available for copying and tinkering in Tinkercad.

If you would like to print the ruler, the STL file is available in Thingiverse: Ruler (mm) Millimeter