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Jacobs Taper Chart: Drill Chuck Taper Dimensions

Guide · Updated October 28, 2026

A Jacobs taper (JT) is the short, self-holding cone that holds a drill chuck on its arbor. The other end of that arbor is usually a Morse taper, R8 or straight shank. Unlike Morse or Jarno tapers, the Jacobs family has no single rate of taper: the angle per side runs from about 1.4° to 2.3° depending on size, so you must identify the exact number before buying an arbor. Dimensions below are the commonly published Jacobs figures (as tabulated in Machinery's Handbook-derived references and the Wikipedia machine taper table); taper per foot and angle are calculated from those diameters and lengths.

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Step by step

Remove the chuck: Use wedges or a chuck removal tool behind the chuck body, never a hammer on the jaws.
Measure the large end: Mic the arbor at the shoulder end of the cone, or the socket mouth with a telescoping gage.
Measure the small end and length: Mic the small end and measure the cone length with a caliper.
Match the chart: Compare all three numbers; the large end alone separates most sizes.
Clean both cones: Wipe with solvent; any chip or burr stops the taper from seating.
Seat the chuck: Retract the jaws fully and seat with a firm blow from a soft mallet or by pressing.

Jacobs taper dimensions

No. 0: small end 0.2284 in (5.802 mm), large end 0.2500 in (6.350 mm), length 0.4375 in; 0.5914 in/ft, 1.41° per side
No. 1: small end 0.3334 in (8.469 mm), large end 0.3840 in (9.754 mm), length 0.6562 in; 0.9251 in/ft, 2.21° per side
No. 2: small end 0.4876 in (12.386 mm), large end 0.5590 in (14.199 mm), length 0.8750 in; 0.9787 in/ft, 2.34° per side
No. 2 Short: small end 0.4876 in (12.386 mm), large end 0.5488 in (13.940 mm), length 0.7500 in; 0.9786 in/ft, 2.33° per side
No. 2-1/2: small end 0.6250 in (15.875 mm), large end 0.6770 in (17.196 mm), length 1.0550 in; 0.5915 in/ft, 1.41° per side
No. 33: small end 0.5605 in (14.237 mm), large end 0.6240 in (15.850 mm), length 1.0000 in; 0.7620 in/ft, 1.82° per side
No. 6: small end 0.6241 in (15.852 mm), large end 0.6760 in (17.170 mm), length 1.0000 in; 0.6228 in/ft, 1.49° per side
No. 3: small end 0.7461 in (18.951 mm), large end 0.8110 in (20.599 mm), length 1.2188 in; 0.6390 in/ft, 1.53° per side
No. 4: small end 1.0372 in (26.345 mm), large end 1.1240 in (28.550 mm), length 1.6563 in; 0.6289 in/ft, 1.50° per side
No. 5: small end 1.3161 in (33.429 mm), large end 1.4130 in (35.890 mm), length 1.8750 in; 0.6202 in/ft, 1.48° per side

No. 2 Short has the same small end as No. 2 but is shorter, so its large end is smaller. No. 33 and No. 6 sit between No. 2 and No. 3 and are the most common sizes on 1/2 in capacity chucks; No. 6 is typical of heavy-duty 1/2 in chucks, No. 33 of lighter ones. Always measure rather than trusting capacity, because makers vary.

DIN 238 B tapers

B10: 10.095 mm, the small end of MT1
B12: 12.065 mm, the large end of MT1
B16: 15.733 mm, the small end of MT2
B18: 17.780 mm, the large end of MT2
B22: 21.793 mm, the small end of MT3
B24: 23.825 mm, the large end of MT3

B tapers are the metric chuck taper family used on most European and many imported chucks. Each one uses the Morse rate of taper (about 1:20), so the number is the large-end diameter rounded to the nearest millimeter. A B16 chuck is very common on 13 mm and 16 mm keyless chucks; B12 on 10 mm chucks.

How to tell JT from B taper

Measure the large end in millimeters. About 15.85 mm is JT33, 17.17 mm is JT6, 14.20 mm is JT2, 15.73 to 17.78 mm points to B16 or B18. Then check the angle: Jacobs tapers are steeper per side (1.8° to 2.3° for the common sizes) than the B series (about 1.43° per side). An arbor that rocks when tried in the socket is the wrong family.

Worked example: identify an unknown arbor

An arbor measures 0.6235 in at the large end, 0.5610 in at the small end and 1.000 in long. Taper per foot = (0.6235 − 0.5610) ÷ 1.000 × 12 = 0.750 in/ft. Angle per side = atan(0.03125 ÷ 1.000) = 1.79°. That matches JT33 (0.6240 / 0.5605 / 1.000) and rules out JT6, whose large end is 0.6760 in.

Worked example: check a socket with a sine bar

For JT6, half-angle is atan((0.6760 − 0.6241) ÷ 2 ÷ 1.000) = 1.487°, included angle 2.974°. On a 5 in sine bar, the gage block stack to set the full included angle against one element is 5 × sin(1.487°) = 0.1297 in. Indicate along the arbor; zero sweep means the angle is right.

Common problems

Chuck falls off: oil or dirt on the taper, or a mismatched size. Chuck wobbles: a burr on the arbor or a bent arbor; check runout at the arbor before blaming the chuck. Taper will not release: use wedges both sides at once. Never use a Jacobs-taper chuck for side milling; the taper is not designed for side load.

FAQ

What is the most common Jacobs taper?

JT33 and JT6 on 1/2 in chucks, and JT2 on 3/8 in chucks.

Is JT33 the same as JT6?

No. JT33 large end is 0.6240 in, JT6 is 0.6760 in.

Is a Jacobs taper the same as a Morse taper?

No. Morse is about 5/8 in per foot; Jacobs angles vary by size.

What is a B16 taper?

The DIN 238 chuck taper equal to the small end of MT2, 15.733 mm.

Do Jacobs tapers need a drawbar?

No. They are self-holding.

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