Type the shank diameter and Toolhelix tells you which ER collet fits in every size, ER8 to ER50, with clamping range and body dimensions. Below is the full chart of the series.
Clamping ranges to DIN 6499 / ISO 15488. The collet maker's catalogue is what counts.
The number after ER is the collet body diameter, rounded: an ER32 has a 33 mm body. Each collet is marked with its range, largest to smallest: an ER32 10–9 holds shanks from 9 to 10 mm. ER collets follow DIN 6499 / ISO 15488 and have an 8° taper: collets and chucks from different makers are interchangeable.
Which size to choose
ER11 and ER16: small tools, short chucks and live tools, where space is tight.
ER20 and ER25: drills and end mills up to 13–16 mm, very common on lathe live tooling.
ER32: the most used of all, up to 20 mm.
ER40 and ER50: large shanks and taps, up to 26 and 34 mm.
For the same shank, a smaller collet takes less space; a larger one grips harder.
Accuracy and clamping
The shank should be close to the collet's maximum and inside along the full collet length: with too little shank inside, runout gets worse and the collet is damaged. Tighten the nut with the chuck maker's wrench and torque, and keep collet and taper clean.
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