Standards of ER Collets These collets are usually found in a standard series as: ER-8, ER-11, ER-16, ER-20, ER-25, ER-32, ER-40, and ER-50. The numbers represent the opening diameter of the tapered receptacle in millimeters.

ER Collets The standard series are: ER-8, ER-11, ER-16, ER-20, ER-25, ER-32, ER-40, and ER-50. ER collets collapse to hold parts up to 1mm smaller than the nominal collet internal size in most of the series (up to 2mm smaller in ER-50, and 0.5mm in smaller sizes) and are available in 1mm or 0.5mm steps.

Your first consideration should be the size of end mills or drills you will be using most often. If you are doing smaller work you would require smaller diameter range collets. Generally you may prefer the ER16 and ER32 sizes. ​If you are doing very small work then perhaps an ER11 set would be the best choice.

5C Collets- Hardened and Ground Collets in round sizes are 1/16" to 1-1/16" in 1/64" increments, or 3mm to 26mm in 1mm increments; 5c square and hex collet sizes are 1/8" to 11/16" in 1/16" increments.