The standard sizes are: ER-8, ER-11, ER-16, ER-20, ER-25, ER-32, ER-40, and ER-50.
ER collets contract over a range of 1mm and are available in 1mm or 0.5mm steps, so a range of ER collets can hold any cylindrical shank, metric or imperial. ER collets may also be used on a lathe to hold work pieces.