Actual 0.5mm thin wall 20 mm box
by Jerry_XYZ, published
This is a calibration object, similar to other thin wall box objects. The walls of this model really are 0.5mm thick. Despite their name, similar objects previously published here apparently have 0.25 mm thick walls. With a 0.5mm nozzle, it seems reasonable to try printing walls actually 0.5mm thick.
Print this and measure the walls to verify that your printer makes walls the correct thickness. Adjust your slicer program's extrusion factors to get the correct wall thickness.
More detailed calibration guidelines have already been posted by people much better at this than I am :)