Key size is chosen from the shaft diameter. In inch (ANSI B17.1), a 1 in shaft takes a 1/4 × 1/4 in square key; in metric (DIN 6885-1), a 25 mm shaft takes an 8 × 7 mm key with a 4.0 mm shaft keyseat and a 3.3 mm hub keyway.

This chart lists the standard parallel key for each shaft diameter range, with the keyseat (shaft) and keyway (hub) depths. Read across from your shaft diameter: the shaft range is "over" the first value up to and including the second. The ANSI table gives square keys and the thinner rectangular (flat) keys; depths at the side wall are half the key height. The DIN table gives the depths t1 and t2 directly, measured from the shaft surface and bore surface at the side of the key. The calculator works out the exact dimensions to measure from the opposite side of the shaft or bore, which is how keyseats are usually checked with a micrometer or caliper. Woodruff key numbers are decoded at the end.

Keyseat depth calculator

Inch results use the ANSI B17.1 square key; C is the top clearance in the hub (0.005 in is typical for parallel keys).

ANSI B17.1 inch keys by shaft diameter

Shaft diameter (over – to incl.)Square key W × HSquare: depth at side, shaft and hub (H/2)Rectangular key W × HRectangular: depth at side (H/2)
5/16 – 7/163/32 × 3/323/64 (.0469)square only–
7/16 – 9/161/8 × 1/81/16 (.0625)1/8 × 3/323/64 (.0469)
9/16 – 7/83/16 × 3/163/32 (.0938)3/16 × 1/81/16 (.0625)
7/8 – 1-1/41/4 × 1/41/8 (.1250)1/4 × 3/163/32 (.0938)
1-1/4 – 1-3/85/16 × 5/165/32 (.1562)5/16 × 1/41/8 (.1250)
1-3/8 – 1-3/43/8 × 3/83/16 (.1875)3/8 × 1/41/8 (.1250)
1-3/4 – 2-1/41/2 × 1/21/4 (.2500)1/2 × 3/83/16 (.1875)
2-1/4 – 2-3/45/8 × 5/85/16 (.3125)5/8 × 7/167/32 (.2188)
2-3/4 – 3-1/43/4 × 3/43/8 (.3750)3/4 × 1/21/4 (.2500)
3-1/4 – 3-3/47/8 × 7/87/16 (.4375)7/8 × 5/85/16 (.3125)
3-3/4 – 4-1/21 × 11/2 (.5000)1 × 3/43/8 (.3750)
4-1/2 – 5-1/21-1/4 × 1-1/45/8 (.6250)1-1/4 × 7/87/16 (.4375)
5-1/2 – 6-1/21-1/2 × 1-1/23/4 (.7500)1-1/2 × 11/2 (.5000)
6-1/2 – 7-1/21-3/4 × 1-3/47/8 (.8750)1-3/4 × 1-1/23/4 (.7500)
7-1/2 – 92 × 21 (1.0000)2 × 1-1/23/4 (.7500)
9 – 112-1/2 × 2-1/21-1/4 (1.2500)2-1/2 × 1-3/47/8 (.8750)
11 – 133 × 31-1/2 (1.5000)3 × 21 (1.0000)
13 – 153-1/2 × 3-1/21-3/4 (1.7500)3-1/2 × 2-1/21-1/4 (1.2500)

Depths are nominal, measured at the side wall of the keyseat. The hub keyway is cut H/2 plus a small top clearance. Inch dimensions in brackets are decimals; mm values are × 25.4.

DIN 6885-1 (ISO 773) metric parallel keys

Shaft d (mm)Key b × h (mm)Shaft depth t1 (mm)Hub depth t2 (mm)t1, t2 toleranceShaft: d − t1Hub: d + t2
over 6 to 82 × 21.21.0+0.1d − 1.2d + 1.0
over 8 to 103 × 31.81.4+0.1d − 1.8d + 1.4
over 10 to 124 × 42.51.8+0.1d − 2.5d + 1.8
over 12 to 175 × 53.02.3+0.1d − 3.0d + 2.3
over 17 to 226 × 63.52.8+0.1d − 3.5d + 2.8
over 22 to 308 × 74.03.3+0.2d − 4.0d + 3.3
over 30 to 3810 × 85.03.3+0.2d − 5.0d + 3.3
over 38 to 4412 × 85.03.3+0.2d − 5.0d + 3.3
over 44 to 5014 × 95.53.8+0.2d − 5.5d + 3.8
over 50 to 5816 × 106.04.3+0.2d − 6.0d + 4.3
over 58 to 6518 × 117.04.4+0.2d − 7.0d + 4.4
over 65 to 7520 × 127.54.9+0.2d − 7.5d + 4.9
over 75 to 8522 × 149.05.4+0.2d − 9.0d + 5.4
over 85 to 9525 × 149.05.4+0.2d − 9.0d + 5.4
over 95 to 11028 × 1610.06.4+0.2d − 10.0d + 6.4
over 110 to 13032 × 1811.07.4+0.2d − 11.0d + 7.4
over 130 to 15036 × 2012.08.4+0.2d − 12.0d + 8.4
over 150 to 17040 × 2213.09.4+0.2d − 13.0d + 9.4
over 170 to 20045 × 2515.010.4+0.2d − 15.0d + 10.4
over 200 to 23050 × 2817.011.4+0.2d − 17.0d + 11.4

Keyway width tolerances (ISO / DIN)

FitShaft keyway widthHub keyway widthUse
NormalN9JS9General use; key fitted firmly in the shaft, hub can be assembled by hand or light press
Tight (close)P9P9Reversing or shock loads, no backlash
Free (sliding)H9D10Hub slides along the key (feather key)

The key itself is made to h9 on width and h11 on height (h9 for square keys). Standard DIN 6885 key lengths (mm): 6, 8, 10, 12, 14, 16, 18, 20, 22, 25, 28, 32, 36, 40, 45, 50, 56, 63, 70, 80, 90, 100, 110, 125, 140, 160, 180, 200, 220, 250. Use the ISO Tolerance Fit Calculator to turn N9, JS9 or P9 into limits in mm.

Woodruff keys (ANSI B17.2): reading the key number

A Woodruff key is a half-moon key that sits in a round-bottomed seat cut with a Woodruff cutter. Its number gives the size: the last two digits are the nominal diameter in eighths of an inch and the digits in front are the width in thirty-seconds. So a No. 405 key is 4/32 = 1/8 in wide on a 5/8 in diameter, and a No. 1210 is 3/8 in wide on a 1-1/4 in diameter. Key height and the seat depth for each number are tabulated in ANSI B17.2; check them there or in the cutter maker's catalogue before cutting.

Key No.WidthNominal diameter
2041/16 in1/2 in
3043/32 in1/2 in
3053/32 in5/8 in
4041/8 in1/2 in
4051/8 in5/8 in
4061/8 in3/4 in
5055/32 in5/8 in
5065/32 in3/4 in
5075/32 in7/8 in
6063/16 in3/4 in
6073/16 in7/8 in
6083/16 in1 in
6093/16 in1-1/8 in
8071/4 in7/8 in
8081/4 in1 in
8091/4 in1-1/8 in
8101/4 in1-1/4 in
8111/4 in1-3/8 in
8121/4 in1-1/2 in
10085/16 in1 in
10095/16 in1-1/8 in
10105/16 in1-1/4 in
10115/16 in1-3/8 in
10125/16 in1-1/2 in
12103/8 in1-1/4 in
12113/8 in1-3/8 in
12123/8 in1-1/2 in

How to use these charts

  • Pick the key from the shaft diameter. If the hub is short or the torque is high, check the key length for shear and crushing; a common rule of thumb is a key length of about 1 to 1.5 × the shaft diameter.
  • Cut the shaft keyseat with an end mill or keyseat cutter of the key width, to depth t1 (metric) or H/2 at the side (inch). Measure over the bottom of the keyseat to the far side of the shaft: that is d − t1, or S from the calculator.
  • Cut the hub keyway by broaching, slotting or wire EDM to depth t2 (metric) or the T dimension (inch), measured from the far side of the bore.
Worked example: a 35 mm shaft is in the over 30 to 38 mm range, so use a 10 × 8 mm key. Cut the shaft keyseat 10 N9 wide and 5.0 mm deep (measure 35 − 5.0 = 30.0 mm over the keyseat), and the hub keyway 10 JS9 wide and 3.3 mm deep (35 + 3.3 = 38.3 mm from the far side of the bore). In inch, a 1-1/2 in shaft takes a 3/8 × 3/8 in key; the calculator gives S ≈ 1.2887 in and T ≈ 1.6687 in with C = 0.005 in.

Frequently asked questions

A 1 in shaft is in the over 7/8 to 1-1/4 in range of ANSI B17.1, so it takes a 1/4 × 1/4 in square key, or a 1/4 × 3/16 in rectangular key. The keyseat depth at the side wall is 1/8 in for the square key.

A 25 mm shaft falls in the over 22 to 30 mm range of DIN 6885-1, so it takes an 8 × 7 mm parallel key. The shaft keyseat depth t1 is 4.0 mm and the hub keyway depth t2 is 3.3 mm.

For an inch square key, the shaft keyseat is cut to half the key height at the side wall and the hub keyway to half the height plus a small clearance. For DIN 6885-1 metric keys, use t1 for the shaft and t2 for the hub from the table, for example 5.0 mm and 3.3 mm for a 10 × 8 key.

They are ISO tolerance zones for the keyway width. N9 in the shaft with JS9 in the hub is the normal fit for general use. P9 in both shaft and hub is a tight fit for reversing or shock loads. H9 in the shaft with D10 in the hub gives a free fit for a sliding hub.

The last two digits are the nominal diameter in eighths of an inch and the digits before them are the width in thirty-seconds of an inch. A No. 606 key is 6/32 = 3/16 in wide with a 6/8 = 3/4 in diameter.

Sources: ANSI B17.1 (keys and keyseats) for inch key sizes and the S/T keyseat formulas, DIN 6885-1 / ISO R773 for metric parallel keys and depths, ANSI B17.2 for the Woodruff key numbering. Keys above 15 in (inch) and 230 mm (metric) shafts are not listed. Always check critical values against the current standard or the key manufacturer.