Parallel key sizing runs one way: the shaft diameter decides the key section, and the key section decides both keyway depths. You do not choose a key size to suit a torque - you take the standard section for that shaft, then check it carries the torque and lengthen it if it does not.
How to Use the Chart
- Measure the shaft diameter.
- Find the row whose range contains it. Ranges are over ... up to and including - a 30 mm shaft falls in the 22 – 30 row, so it takes an 8 × 7 key, not 10 × 8.
- Read across for the key section b × h, the shaft keyway depth t₁ and the hub keyway depth t₂.
Both depths are measured from the surface down to the bottom of the slot, on the centreline. They add up to slightly more than the key height, and that small surplus is deliberate - it is the top clearance that stops the key binding on its top face, so that the drive runs through the key's sides, where it should.
Key Size Chart
| Shaft Ø (over - to) | Key b × h | Shaft keyway t₁ | Hub keyway t₂ | Key length range |
|---|---|---|---|---|
| 6 – 8 | 2 × 2 | 1.2 | 1.0 | 6 – 20 |
| 8 – 10 | 3 × 3 | 1.8 | 1.4 | 6 – 36 |
| 10 – 12 | 4 × 4 | 2.5 | 1.8 | 8 – 45 |
| 12 – 17 | 5 × 5 | 3.0 | 2.3 | 10 – 56 |
| 17 – 22 | 6 × 6 | 3.5 | 2.8 | 14 – 70 |
| 22 – 30 | 8 × 7 | 4.0 | 3.3 | 18 – 90 |
| 30 – 38 | 10 × 8 | 5.0 | 3.3 | 22 – 110 |
| 38 – 44 | 12 × 8 | 5.0 | 3.3 | 28 – 140 |
| 44 – 50 | 14 × 9 | 5.5 | 3.8 | 36 – 160 |
| 50 – 58 | 16 × 10 | 6.0 | 4.3 | 45 – 180 |
| 58 – 65 | 18 × 11 | 7.0 | 4.4 | 50 – 200 |
| 65 – 75 | 20 × 12 | 7.5 | 4.9 | 56 – 220 |
| 75 – 85 | 22 × 14 | 9.0 | 5.4 | 63 – 250 |
| 85 – 95 | 25 × 14 | 9.0 | 5.4 | 70 – 280 |
| 95 – 110 | 28 × 16 | 10.0 | 6.4 | 80 – 320 |
| 110 – 130 | 32 × 18 | 11.0 | 7.4 | 90 – 360 |
| 130 – 150 | 36 × 20 | 12.0 | 8.4 | 100 – 400 |
| 150 – 170 | 40 × 22 | 13.0 | 9.4 | 100 – 400 |
| 170 – 200 | 45 × 25 | 15.0 | 10.4 | 110 – 450 |
| 200 – 230 | 50 × 28 | 17.0 | 11.4 | 125 – 500 |
| 230 – 260 | 56 × 32 | 20.0 | 12.4 | 140 – 500 |
| 260 – 290 | 63 × 32 | 20.0 | 12.4 | 160 – 500 |
| 290 – 330 | 70 × 36 | 22.0 | 14.4 | 180 – 500 |
| 330 – 380 | 80 × 40 | 25.0 | 15.4 | 200 – 500 |
| 380 – 440 | 90 × 45 | 28.0 | 17.4 | 220 – 500 |
| 440 – 500 | 100 × 50 | 31.0 | 19.5 | 250 – 500 |
All dimensions in mm. Shaft ranges are "over ... up to and including" - a 30 mm shaft takes the 8 × 7 key, not 10 × 8. Depths t₁ and t₂ are measured from the surface to the bottom of the slot on the centreline.
Worked Example
A 45 mm shaft driving a pulley:
- 45 mm falls in the 44 – 50 range → key section 14 × 9 mm.
- Shaft keyway: 14 mm wide, 5.5 mm deep. Hub keyway: 14 mm wide, 3.8 mm deep.
- Check: 5.5 + 3.8 = 9.3 mm against a 9 mm key - 0.3 mm of top clearance, as intended.
- The shaft diameter under the keyway is 45 − 5.5 = 39.5 mm, which is the figure to use for shaft strength calculations.
The keyway weakens the shaft. Cutting a 5.5 mm slot into a 45 mm shaft removes material where bending stress is highest and adds a stress concentration at the keyway corners. Size shaft strength on the section under the keyway, not the nominal diameter.
Types A, B and C
The section is the same; only the ends differ:
- Type A - both ends rounded. For keyways cut with an end mill, which leaves a round-ended pocket. The most common in general machinery.
- Type B - both ends square. For keyways cut with a side-and-face cutter or planed through, running the full length of the shaft.
- Type C - one round, one square. For a keyway that runs out to the end of the shaft at one end and is pocketed at the other.
Key steel bought in bar form - the way most workshops stock it - is square-ended, and you cut it to length and radius the ends yourself if the keyway needs Type A.
Tolerances and Fits
The width b is the working dimension - it transmits the torque, and it is the only one with a meaningful fit class:
- Free fit (shaft H9, hub D10): the hub can slide along the key. Use where a gear or coupling must move axially.
- Normal fit (shaft N9, hub JS9): the general-purpose fit for a fixed hub. This is what most drawings mean when they specify nothing.
- Close fit (shaft P9, hub P9): an interference fit, for reversing drives and shock loading where any backlash would hammer the keyway.
The key height h is not a fit dimension - the top clearance means it never bears. A key that is tight on its top face is fitted wrong, and will spread the load onto the corners.
Choosing the Length
Length is the variable you actually have control over, and it is how you match a standard section to your torque. A common starting point is a key length of roughly 1.5 times the shaft diameter, then check it:
- Shear: the key can shear across its width along the shaft interface.
- Crushing: the key side bears against the keyway wall. This usually governs, because only about half the key height is bearing on each side.
If the key is not long enough and the hub is not either, the usual answer is two keys at 120° - not 180°, which weakens the shaft on a single diameter - or a splined connection instead.
Mistakes to Avoid
- Reading the shaft range wrong at a boundary. The ranges are inclusive of the upper figure. A 22 mm shaft takes 6 × 6; a 22.5 mm shaft takes 8 × 7.
- Measuring keyway depth to the wrong place. t₁ and t₂ are to the bottom of the slot on the centreline, not the shoulder depth at the edge.
- Fitting the key tight on the top. If it needs driving in vertically, the hub keyway is too shallow - the top clearance has been machined out.
- Assuming imported equipment is metric. Inch key sections are close to, but not the same as, metric ones. A 1/4 inch key is 6.35 mm and will be sloppy in a 6 mm keyway and will not enter a 5 mm one.
- Using the nominal diameter for shaft strength. Always calculate on the reduced section under the keyway.
Need Keys Cut to Length?
Parallel keys in EN-8 steel and stainless, metric and inch sections, supplied in 300 mm lengths or cut to your size.
View Key RangeFrequently Asked Questions
What size key does a 25 mm shaft need?
A 25 mm shaft falls in the 22 – 30 mm range, so it takes an 8 × 7 mm parallel key. The shaft keyway is 8 mm wide and 4.0 mm deep; the hub keyway is 8 mm wide and 3.3 mm deep.
How do I read keyway depths t1 and t2?
Both are measured from the surface to the bottom of the slot on the centreline - t₁ into the shaft, t₂ into the hub. They sum to slightly more than the key height, and that surplus of 0.2 to 0.5 mm is deliberate top clearance so the key drives through its sides rather than binding on its top face.
What is the difference between Type A, B and C parallel keys?
Only the ends. Type A has both ends rounded, to suit a keyway cut with an end mill. Type B has both ends square, for keyways cut with a side-and-face cutter or run through. Type C has one rounded and one square end. The section dimensions are identical across all three.
Are DIN 6885 and IS 2048 parallel keys interchangeable?
For the standard metric sections the width, height and keyway depths agree, so keys made to either standard interchange in practice. Confirm the end type and the fit class on the drawing, since those are specified independently of the section.
How long should a parallel key be?
A common starting point is about 1.5 times the shaft diameter, then check it against both shear across the key and crushing on the keyway side face. Crushing usually governs, because only around half the key height bears on each side. If one key is not enough, fit two at 120° rather than 180°, which would weaken the shaft on a single diameter.
Does a keyway weaken the shaft?
Yes, in two ways: it removes material where bending stress is highest, and the keyway corners are a stress concentration. Always calculate shaft strength on the reduced section under the keyway - for a 45 mm shaft with a 5.5 mm deep keyway, that is 39.5 mm, not 45 mm.