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.

As you tighten the nut on your holder, the ER collet collapses together to hold round tools or stock from 0.5-2 mm (or 0.0197-0.0787 in., depending on collet size) smaller than the collet internal diameter. While the initial ER system is based in metric, there are ER collets available with imperial sizing as well.