The calibers of small arms (weapons with a caliber below 20 millimeters) are measured by the Germans in millimeters, whereas weapons with calibers of 20 millimeters and higher are measured in centimeters: for example, the 9-mm Pistole 08, the 7.5-cm Kanone. This should not cause any confusion, as the conversion from centimeters to millimeters is simply a multiplication by 10.

Some confusion, however, may be caused by the German practice of referring to calibers approximately. The Germans refer to their rifle and machine-gun ammunition as caliber 7.9 mm, whereas in reality it is caliber 7.92 mm (.312 inch). Their 8-cm mortar ammunition is actually 8.1 cm. In many instances the exact caliber is used: for example, 3.7-cm Pak.