Standard parallel key and keyway sizes for shafts from 6 mm to 500 mm, to BS 4235-1, ISO 773 and DIN 6885-1, which share the same dimensions. The shaft diameter sets the key: a 25 mm shaft takes an 8 x 7 mm key with a keyway 4 mm deep in the shaft and 3.3 mm deep in the hub, a 40 mm shaft takes a 12 x 8 mm key and a 50 mm shaft takes a 14 x 9 mm key.
Metric keyway dimensions by shaft diameter
Find the band your shaft diameter falls in. Each band runs from over the first figure up to and including the second. The keyway is the same width as the key, and the two depths add up to slightly more than the key height so the key has clearance at the top.
| Shaft diameter, over to up to (mm) |
Key b x h (mm) |
Keyway width b (mm) |
Depth in shaft t1 (mm) |
Depth in hub t2 (mm) |
Corner radius max (mm) |
| 6 to 8 | 2 x 2 | 2 | 1.2 | 1 | - |
| 8 to 10 | 3 x 3 | 3 | 1.8 | 1.4 | - |
| 10 to 12 | 4 x 4 | 4 | 2.5 | 1.8 | 0.16 |
| 12 to 17 | 5 x 5 | 5 | 3 | 2.3 | 0.25 |
| 17 to 22 | 6 x 6 | 6 | 3.5 | 2.8 | 0.25 |
| 22 to 30 | 8 x 7 | 8 | 4 | 3.3 | 0.25 |
| 30 to 38 | 10 x 8 | 10 | 5 | 3.3 | 0.4 |
| 38 to 44 | 12 x 8 | 12 | 5 | 3.3 | 0.4 |
| 44 to 50 | 14 x 9 | 14 | 5.5 | 3.8 | 0.4 |
| 50 to 58 | 16 x 10 | 16 | 6 | 4.3 | 0.4 |
| 58 to 65 | 18 x 11 | 18 | 7 | 4.4 | 0.4 |
| 65 to 75 | 20 x 12 | 20 | 7.5 | 4.9 | 0.6 |
| 75 to 85 | 22 x 14 | 22 | 9 | 5.4 | 0.6 |
| 85 to 95 | 25 x 14 | 25 | 9 | 5.4 | 0.6 |
| 95 to 110 | 28 x 16 | 28 | 10 | 6.4 | 0.6 |
| 110 to 130 | 32 x 18 | 32 | 11 | 7.4 | 0.6 |
| 130 to 150 | 36 x 20 | 36 | 12 | 8.4 | 1 |
| 150 to 170 | 40 x 22 | 40 | 13 | 9.4 | 1 |
| 170 to 200 | 45 x 25 | 45 | 15 | 10.4 | 1 |
| 200 to 230 | 50 x 28 | 50 | 17 | 11.4 | 1 |
| 230 to 260 | 56 x 32 | 56 | 20 | 12.4 | - |
| 260 to 290 | 63 x 32 | 63 | 20 | 12.4 | - |
| 290 to 330 | 70 x 36 | 70 | 22 | 14.4 | - |
| 330 to 380 | 80 x 40 | 80 | 25 | 15.4 | - |
| 380 to 440 | 90 x 45 | 90 | 28 | 17.4 | - |
| 440 to 500 | 100 x 50 | 100 | 31 | 19.5 | - |
How keyway depth is measured
Depth in the shaft, t1, is measured from the shaft surface at the keyway centreline down to the keyway floor. Depth in the hub, t2, is measured from the bore surface up into the hub. On a 25 mm shaft, t1 is 4 mm and t2 is 3.3 mm, which together make 7.3 mm against a key height of 7 mm. The difference is the clearance above the key: a parallel key drives on its sides, not on its top.
On a drawing the shaft keyway is usually dimensioned from the opposite side of the shaft, as the diameter minus t1, and the hub keyway as the bore plus t2, because those are the dimensions that can be measured directly.
Keyway width tolerances and fit
The fit of a key is set by the tolerance on the keyway width, not by changing the key. The standard gives three fits:
| Fit |
Shaft keyway width |
Hub keyway width |
| Free | H9 | D10 |
| Normal | N9 | JS9 |
| Close | P9 | P9 |
A free fit lets a hub slide along the shaft under load. A normal fit is the general purpose choice for a hub that is assembled once and stays put. A close fit is for reversing or shock loads, where any side clearance would let the key hammer in the keyway.
Imperial keyway sizes (BS 46)
Older British machinery uses inch keys to BS 46 Part 1, which has been withdrawn but is still needed for repair work. It lists both square and rectangular keys. Depths are shown as the minimum to maximum.
| Key type |
Shaft diameter, over to up to (in) |
Key width x height (in) |
Depth in shaft (in) |
Depth in hub (in) |
| Square | 1/4 to 1/2 | 1/8 x 1/8 | 0.072 to 0.078 | 0.060 to 0.066 |
| Square | 1/2 to 3/4 | 3/16 x 3/16 | 0.107 to 0.113 | 0.088 to 0.094 |
| Square | 3/4 to 1 | 1/4 x 1/4 | 0.142 to 0.148 | 0.115 to 0.121 |
| Square | 1 to 1 1/4 | 5/16 x 5/16 | 0.177 to 0.183 | 0.142 to 0.148 |
| Square | 1 1/4 to 1 1/2 | 3/8 x 3/8 | 0.213 to 0.219 | 0.169 to 0.175 |
| Square | 1 1/2 to 1 3/4 | 7/16 x 7/16 | 0.248 to 0.254 | 0.197 to 0.203 |
| Square | 1 3/4 to 2 | 1/2 x 1/2 | 0.283 to 0.289 | 0.224 to 0.230 |
| Square | 2 to 2 1/2 | 5/8 x 5/8 | 0.354 to 0.360 | 0.278 to 0.284 |
| Square | 2 1/2 to 3 | 3/4 x 3/4 | 0.424 to 0.430 | 0.333 to 0.339 |
| Square | 3 to 3 1/2 | 7/8 x 7/8 | 0.495 to 0.501 | 0.387 to 0.393 |
| Square | 3 1/2 to 4 | 1 x 1 | 0.566 to 0.572 | 0.442 to 0.448 |
| Square | 4 to 5 | 1 1/4 x 1 1/4 | 0.707 to 0.713 | 0.551 to 0.557 |
| Square | 5 to 6 | 1 1/2 x 1 1/2 | 0.848 to 0.854 | 0.661 to 0.667 |
| Rectangular | 1 to 1 1/4 | 5/16 x 1/4 | 0.146 to 0.152 | 0.112 to 0.118 |
| Rectangular | 1 1/4 to 1 1/2 | 3/8 x 1/4 | 0.150 to 0.156 | 0.108 to 0.114 |
| Rectangular | 1 1/2 to 1 3/4 | 7/16 x 5/16 | 0.186 to 0.192 | 0.135 to 0.141 |
| Rectangular | 1 3/4 to 2 | 1/2 x 5/16 | 0.190 to 0.196 | 0.131 to 0.137 |
| Rectangular | 2 to 2 1/2 | 5/8 x 7/16 | 0.260 to 0.266 | 0.185 to 0.191 |
| Rectangular | 2 1/2 to 3 | 3/4 x 1/2 | 0.299 to 0.305 | 0.209 to 0.215 |
| Rectangular | 3 to 3 1/2 | 7/8 x 5/8 | 0.370 to 0.376 | 0.264 to 0.270 |
| Rectangular | 3 1/2 to 4 | 1 x 3/4 | 0.441 to 0.447 | 0.318 to 0.324 |
| Rectangular | 4 to 5 | 1 1/4 x 7/8 | 0.518 to 0.524 | 0.366 to 0.372 |
| Rectangular | 5 to 6 | 1 1/2 x 1 | 0.599 to 0.605 | 0.412 to 0.418 |
Choosing key length
The table fixes the key cross-section but not its length. Length is chosen so the key carries the torque without crushing the keyway sides. A very long key does not load evenly along its length, so if a key of sensible length is not enough, the usual answer is a larger shaft, a splined connection or an interference fit, not a longer key.
References
- BS 4235-1 - Metric keys and keyways, parallel and taper keys
- ISO 773 - Rectangular or square parallel keys and their corresponding keyways
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