ER collets run from ER8 (0.5 to 5 mm shanks) up to ER50 (up to 34 mm), and each collet grips a 1 mm range below its marked size (0.5 mm on ER8 and ER11), so an ER32 10 mm collet holds 9.0 to 10.0 mm.

This chart gives the clamping range, collet outside diameter and length, nut thread and typical nut tightening torque for every common ER series, plus a quick lookup that tells you which collet to grab for a metric or inch shank. The number after "ER" is roughly the collet's outside diameter in millimetres. All ER collets share the same 8° (16° included) taper and double-slotted design from REGO-FIX, standardised as DIN 6499 and now ISO 15488, so any maker's collet fits any holder of the same size. Use the Metric / Inch switch to change units.

ER collet size chart

SeriesClamping rangesmallest to largest colletRange per colletCollet OD (D)Collet length (L)Nut threadNut torquestandard hex nut, approx.Typical use
ER80.5 – 5 mm0.020 – 0.197 in0.5 mm0.020 in8.5 mm0.335 in13.5 mm0.531 inM12 x 0.75see nut makerEngraving, micro drills and small high-speed spindles
ER110.5 – 7 mm0.020 – 0.276 in0.5 mm0.020 in11.5 mm0.453 in18 mm0.709 inM14 x 0.7524 N·m18 ft·lbSmall end mills and drills, router and Swiss-type tooling
ER160.5 – 10 mm0.020 – 0.394 in1 mm0.039 in17 mm0.669 in27.5 mm1.083 inM22 x 1.556 N·m41 ft·lbCompact tool holders, live tools and gang tooling
ER201 – 13 mm0.039 – 0.512 in1 mm0.039 in21 mm0.827 in31.5 mm1.240 inM25 x 1.580 N·m59 ft·lbGeneral small milling and drilling, lathe live tools
ER251 – 16 mm0.039 – 0.630 in1 mm0.039 in26 mm1.024 in34 mm1.339 inM32 x 1.5104 N·m77 ft·lbMid-size mills and routers, 1/2 in and 5/8 in shanks
ER322 – 20 mm0.079 – 0.787 in1 mm0.039 in33 mm1.299 in40 mm1.575 inM40 x 1.5136 N·m100 ft·lbThe most common size for VMC holders, lathe chucks and fixtures
ER403 – 26 mm0.118 – 1.024 in1 mm0.039 in41 mm1.614 in46 mm1.811 inM50 x 1.5176 N·m130 ft·lbLarger cutters and taps up to 1 in, lathe collet chucks
ER506 – 34 mm0.236 – 1.339 in1 mm0.039 in52 mm2.047 in60 mm2.362 inM64 x 2see nut makerHeavy work holding and large shanks on big spindles and lathes

Collet dimensions are nominal values per ISO 15488 / DIN 6499 as listed by REGO-FIX. The smallest collet in each series varies by maker, so the lower end of the clamping range is typical. Torques are REGO-FIX recommendations for standard hex nuts at the larger bores and are approximate; ER8 and ER50 torques are left out here, so use the nut maker's figure. Check critical values against the standard or your holder maker's catalogue.

How the clamping range works

An ER collet is a spring steel sleeve with slots cut alternately from each end, so it can close evenly over its whole length. That gives each collet a working range instead of a single size: ER16 to ER50 collets close 1 mm below their nominal size and ER8 and ER11 collets close 0.5 mm. A collet marked 12 grips 11.0 to 12.0 mm; a set in 1 mm steps therefore covers every size in between with no gaps.

Worked example. You need to hold a 1/2 in end mill in an ER32 holder. 1/2 in = 12.7 mm, which sits in the 12.0 to 13.0 mm range of the 13 mm collet. A 12 mm collet would be too small; a 14 mm collet would have to close 1.3 mm, beyond its range, and would grip badly.

Grip and runout are best when the shank is at or just under the collet's nominal size, where the collet closes the least. A shank right at the bottom of the range still holds, but the collet bends more and is less accurate, so when two collets overlap pick the one closest above the shank.

Which collet for my shank?

Type a shank diameter to see which collet to use in every ER series that can hold it.

ShankShank in mmER8 / ER11 collet0.5 mm rangeER16 to ER50 collet1 mm rangeSeries that hold it
3 mm3.000 mm3 mm grips 2.5–33 mm grips 2–3ER8, ER11, ER16, ER20, ER25, ER32, ER40
1/8 in3.175 mm3.5 mm grips 3–3.54 mm grips 3–4ER8, ER11, ER16, ER20, ER25, ER32, ER40
4 mm4.000 mm4 mm grips 3.5–44 mm grips 3–4ER8, ER11, ER16, ER20, ER25, ER32, ER40
3/16 in4.763 mm5 mm grips 4.5–55 mm grips 4–5ER8, ER11, ER16, ER20, ER25, ER32, ER40
5 mm5.000 mm5 mm grips 4.5–55 mm grips 4–5ER8, ER11, ER16, ER20, ER25, ER32, ER40
6 mm6.000 mm6 mm grips 5.5–66 mm grips 5–6ER11, ER16, ER20, ER25, ER32, ER40, ER50
1/4 in6.350 mm6.5 mm grips 6–6.57 mm grips 6–7ER11, ER16, ER20, ER25, ER32, ER40, ER50
5/16 in7.938 mmtoo large8 mm grips 7–8ER16, ER20, ER25, ER32, ER40, ER50
8 mm8.000 mmtoo large8 mm grips 7–8ER16, ER20, ER25, ER32, ER40, ER50
3/8 in9.525 mmtoo large10 mm grips 9–10ER16, ER20, ER25, ER32, ER40, ER50
10 mm10.000 mmtoo large10 mm grips 9–10ER16, ER20, ER25, ER32, ER40, ER50
12 mm12.000 mmtoo large12 mm grips 11–12ER20, ER25, ER32, ER40, ER50
1/2 in12.700 mmtoo large13 mm grips 12–13ER20, ER25, ER32, ER40, ER50
14 mm14.000 mmtoo large14 mm grips 13–14ER25, ER32, ER40, ER50
5/8 in15.875 mmtoo large16 mm grips 15–16ER25, ER32, ER40, ER50
16 mm16.000 mmtoo large16 mm grips 15–16ER25, ER32, ER40, ER50
18 mm18.000 mmtoo large18 mm grips 17–18ER32, ER40, ER50
3/4 in19.050 mmtoo large20 mm grips 19–20ER32, ER40, ER50
20 mm20.000 mmtoo large20 mm grips 19–20ER32, ER40, ER50
25 mm25.000 mmtoo large25 mm grips 24–25ER40, ER50
1 in25.400 mmtoo large26 mm grips 25–26ER40, ER50
32 mm32.000 mmtoo large32 mm grips 31–32ER50

Nut torque

Collet grip comes from nut torque, so under-tightening lets tools slip and pull out, while over-tightening distorts the collet, ruins runout and can crack the nut. The figures in the chart are for standard hex nuts at the larger bores of each series. REGO-FIX lists much lower torque for small-bore collets (for example about 8 N·m for an ER11 collet of 1 to 2.9 mm) and for slim mini nuts, while ball-bearing nuts give more grip for the same wrench torque. The Collet Torque calculator gives the value for each series and bore.

Shop tips

  • Snap the collet into the nut first. Clip the collet into the nut's eccentric ring before threading it onto the holder; dropping a loose collet into the taper and then fitting the nut can jam it crooked and damage it.
  • Get the shank in deep enough. Insert the shank at least two-thirds of the collet length for full grip; very short engagement bell-mouths the collet.
  • Keep it clean. Wipe the taper, collet and nut thread. A chip in the taper shows up as runout at the tool tip.
  • Use a fixture and torque wrench. Tighten in a tightening fixture or vise-mounted block, not in the spindle, and use the series torque.
  • Seal for through-coolant. Use sealing discs or sealed collets when running coolant through the tool, or most of the pressure escapes through the slots.
  • Retire worn collets. Bell-mouthing, fretting marks or a collet that no longer springs open are signs it is time to replace it.

Frequently asked questions

A standard ER32 collet set covers shanks from about 2 mm to 20 mm (0.08 to 0.79 in). Each individual collet grips a 1 mm range, from 1 mm under its marked size up to the marked size, so a 10 mm ER32 collet holds 9.0 to 10.0 mm.

ER16 to ER50 collets each close down 1 mm below their nominal size, for example an ER25 12 mm collet grips 11.0 to 12.0 mm. ER8 and ER11 collets have a 0.5 mm range. For the best grip and runout choose the collet whose nominal size is equal to or just above the shank diameter.

With a standard hex nut REGO-FIX recommends about 136 N·m (100 ft·lb) for ER32 at the larger bores. Small-bore collets and mini nuts need much less, so use the Collet Torque calculator or the nut maker's table, and always tighten with a torque wrench in a proper fixture, never by feel.

Yes. Pick the metric collet whose range covers the shank: a 1/2 in (12.7 mm) shank goes in a 13 mm collet that grips 12 to 13 mm, and a 1/4 in (6.35 mm) shank goes in a 7 mm collet. Inch-marked ER collets are also sold if you want the shank close to the nominal size.

Yes, the collet and its seat are standardised by ISO 15488 (formerly DIN 6499), so an ER32 collet from any maker fits any ER32 chuck. Nuts are less universal: they must match the holder's thread and the collet's extraction groove, and precision grades differ between makers.