ER20 series collets have a capacity of 1mm-14mm. Each ER20 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, ER20-3mm can grab a round shank from 3mm diameter to 2.1mm diameter.

The standard series are: ER-8, ER-11, ER-16, ER-20, ER-25, ER-32, ER-40, and ER-50. The "ER" name came from an existing "E" collet (which were a letter series of names) which Rego-Fix modified and appended "R" for "Rego-Fix". The series number is the opening diameter of the tapered receptacle, in millimetres.

There's two types of commercially available ER20 collet nuts- one comes with M24x1. 0 threads and the other comes with M25x1. 5 threads.

Our ER20 collets have a capacity ranging from . 0625 - . 5000 inches or in metric sizes from 2mm up to 13mm. The number shown on the collet is the largest size that particular collet can hold, but it can be tightened to a smaller size.