ER25 series collets have a capacity of 1mm-17mm. Each ER25 collet has a range of (1 mm). The size indicated on the collet is the largest size it can hold and can be collapsed smaller within its collapse range of (0.9mm). For example, ER25-3mm can grab a round shank from 3mm diameter to 2.1mm diameter.

An ER collet is a CNC tool used to hold various components used in mills. These devices consist of the main body and a threat end. The nut screws onto the main shaft. ER collets are available in ER-8 through ER-50 series, with each model number referring to the holder's diameter size in millimeters.