ER collet sizes, dimensions and how to pick the right one

A free reference for anyone running ER collets on a CNC router, mill or lathe — every size from ER8 to ER50, with the numbers you actually need before you order.

Jump straight to what you need

Full ER dimension chart (ER8–ER50) — body Ø, length, clamping range, collapse range
ER nut thread chart — which thread fits your holder, and why there are two
R8 collets — drawbar thread, size sets, the 1/2" ceiling
Morse taper dimensions — MT0–MT7, and MTA vs MTB
SK collets — the high-grip series, and why “SK” also means a holder taper
OZ collets — DIN 6388 dimensions and the slot geometry difference
DC / SDC rear-pull collets — no nut, no collapse range
NT30 chucks — drawbar threads, and why NT has no pull stud

Browse by system

Collets

ER collets

The default on CNC routers and mills. Eight series from ER8 to ER50, each covering about 1 mm of shank range.

Nut threads · Spanners

R8

Bridgeport-type knee mills. One size per collet, drawn in from the top of the head with a drawbar.

C power collets

Straight-shank, heavy grip, no collapse range. The answer when a cutter keeps pulling out of an ER.

SK collets

The high-grip series: SK6 to SK25. Longer body and steeper angle than ER, and it needs its own chuck.

OZ collets

DIN 6388 heavy-duty series, OZ16 to OZ50. Slots run from one end only, so it clamps tangentially.

DC / SDC rear-pull collets

No front nut — drawn in from the back. Slimmest nose of any system, and no collapse range at all.

5C collets

The American lathe-workpiece standard. Capacity, threads, and the Hardinge trap that catches buyers out.

TG collets

TG38 to TG200. A 4° seat angle and more slots than ER, so it grips harder — and the nut costs you pocket reach.

Tapping collets

Why a plain ER collet lets a tap slip on reversal, and what the square drive in an ERT collet actually locks.

Tool holders

Tool holder tapers

BT, CAT, SK and NT compared — same taper, different flange, keys and retention. Plus FMB face mill arbors.

Pull stud dimensions

HSK holders

Hollow taper, 1:10 ratio, contact on the taper and the flange face at once. HSK-A to HSK-A125.

NT30 chucks

DIN 2080 manual-change holders. The three drawbar thread families, and why NT has no pull stud.

Morse taper

MT0–MT7 dimensions, how to identify a size from one measurement, and why MTA and MTB are not interchangeable.

Pull studs

BT, CAT and Mazak retention knobs, and why one taper has several different studs.

Shrink-fit holders

No collet and no nut — heat, drop, cool. The slimmest nose on this site, and h6 shanks are not optional.

Hydraulic chucks

Oil pressure squeezes a sleeve 360° around the shank. Best damping here, and less torque than you'd expect.

End mill holders

Weldon flat dimensions, the 3/4" threshold where set-screw retention stops being optional, and power milling chucks.

Jacobs taper / JT

JT0 to JT33 for drill chuck arbors. The numbers are not in size order, and the taper angle changes between sizes.

Extensions

Extension rods & long chucks

Overhang follows a cube law: halve it and the tool gets eight times stiffer. How to spend reach deliberately.

Accessories

Nuts, spanners & torque

ER nut torque by size and by bore band, which spanner fits which nut, and the fitting order that prevents pull-out.

Torque chart · Sealed collets

What an ER collet is

An ER collet is a split sleeve that is squeezed by a tapered nut so it grips a round shank — end mills, drills, reamers, taps. It is defined by DIN 6499 (now also ISO 15488), which is why a collet from one maker will physically fit a holder from another.

The system is popular for three reasons: one holder accepts a wide range of shank sizes, the grip is concentric enough for real cutting, and changing a tool takes seconds with a hook spanner.

The quick answer: which ER size do I need?

Collet size is set by the largest shank you will ever clamp. Don't size to what you use most often — once you own a 20 mm end mill, an ER20 holder cannot hold it.

Collet	Max tool Ø (mm)	Body Ø (mm)	Length (mm)	Typically found on
ER8	5	8.45	13.5	Engraving spindles, small DC motors
ER11	7	11.5	18.0	3018 / small hobby routers, PCB mills
ER16	10	17.0	27.5	Bench mills, small ATC spindles
ER20	13	21.0	31.5	6040 routers, 1.5 kW air-cooled spindles
ER25	16	26.0	34.0	2.2 kW water-cooled spindles, most DIY builds
ER32	20	33.0	40.0	ISO20 / BT30 tool holders, larger routers
ER40	26	41.0	46.0	Vertical machining centres
ER50	34	52.0	60.0	Heavy roughing, large lathes

Body Ø figures are the nominal external diameter of the collet body. Sources agree within rounding; see the source note on the full chart.

Two things people get wrong before they buy

1. The nut is not optional

The collet is useless without a nut whose thread matches the holder — and the same ER number can exist with two different threads. Check the nut chart before ordering either part.

2. One collet = one size

A collet only grips about 1 mm below its nominal size (0.5 mm on ER8/ER11). It is not a drill chuck. If you clamp outside that window you get runout, slipping, and eventually a ruined shank.