ER11 series collets have a capacity of .020-.3125 inches. Each ER11 collet has a range of .020 inches(0.5mm). The size indicated on the collet is the largest size it can hold and can be collapsed smaller within its collapse range of .020 inches(0.5mm). For example, ER11-3mm can grab a round shank from 3mm diameter to 2.5mm diameter. Please refer to our ER11 Collet print for general ER11 dimensions.