Guide
What are the standard keyway dimensions and tolerances for a shaft and hub?
Keyway dimensions are the width, depth and length of the slot cut into a shaft (the keyseat) and into the bore of a mating hub (the keyway), sized together with a parallel key so that torque passes from one to the other without the key rocking, shearing or jamming. In inch work they come from ASME B17.1; in metric work most drawings use DIN 6885-1, which carries the same key-size series as the withdrawn ISO recommendation R773.
By Saola Manufacturing engineering team · Published · Updated
This guide covers how key size follows shaft size, which width fit to call out for a fixed, sliding or tight joint, how to dimension keyseat depth so it can actually be measured, and the drawing mistakes that most often turn a good shaft into a scrapped one. The tables give the common sizes for orientation; take the final limits for your part from your own copy of the standard.
On this page
How do you pick the key size for a shaft diameter?
Read the shaft diameter range in the standard and take the key width and height listed for it. Both ASME B17.1 and DIN 6885-1 tie one key size to each band of shaft diameters, so the shaft picks the key, not the other way round.
In the inch series B17.1 lists both a square key and a rectangular (flat) key for each diameter band; square keys are the usual choice on small and mid-size shafts and rectangular keys on large ones. ASME describes B17.1 as establishing “a uniform relationship between shaft size and key size for parallel and taper keys” and notes that its shaft diameters are listed only to identify key sizes, not to set shaft dimensions or tolerances (ASME B17.1 scope). The scope also limits the tabulated sizes and tolerances to single-key applications, which matters if you plan two keys at 180 degrees.
The metric series in DIN 6885-1 uses rectangular keys almost everywhere above the smallest sizes, so the key is lower than it is wide and the shaft groove is deeper than the hub groove. The current edition is DIN 6885-1:2021-11, which DIN titles “parallel keys, keyways, deep pattern” (DIN Media listing). The standard key for a given diameter is a starting point: check the key’s shear and the side pressure on the hub for your torque, and use a longer hub, a second key or a spline when one standard key is not enough.
| Shaft diameter, over to incl. (in) | Key width W (in) | Key height H, square / rectangular (in) |
|---|---|---|
| 7/16 to 9/16 | 1/8 | 1/8 / 3/32 |
| 9/16 to 7/8 | 3/16 | 3/16 / 1/8 |
| 7/8 to 1-1/4 | 1/4 | 1/4 / 3/16 |
| 1-1/4 to 1-3/8 | 5/16 | 5/16 / 1/4 |
| 1-3/8 to 1-3/4 | 3/8 | 3/8 / 1/4 |
| 1-3/4 to 2-1/4 | 1/2 | 1/2 / 3/8 |
| 2-1/4 to 2-3/4 | 5/8 | 5/8 / 7/16 |
What are the metric keyway dimensions in DIN 6885-1?
Each DIN 6885-1 size gives a key width b and height h, a shaft groove depth t1 and a hub groove depth t2, all in millimeters. The table below lists the sizes most often seen on general machinery shafts.
Because t1 and t2 are measured from the shaft surface and the bore surface respectively, they add up to slightly more than the key height, which leaves a small clearance over the top of the key in the hub. That top clearance is deliberate: a parallel key drives on its sides, and a key that bottoms in both grooves at once makes the hub hard to assemble and can lift it off center.
Both depth values carry a plus-only tolerance in DIN 6885-1, so the groove may be cut slightly deeper than nominal but never shallower. Copy that one-sided tolerance onto the drawing rather than converting it to a plus-and-minus value; a symmetric tolerance around nominal allows a shallow groove, which eats into the small top clearance and, at the limit, lets the key bottom in the hub keyway.
| Shaft diameter d, over to incl. (mm) | Key b × h (mm) | Shaft depth t1 / hub depth t2 (mm) |
|---|---|---|
| 10 to 12 | 4 × 4 | 2.5 / 1.8 |
| 12 to 17 | 5 × 5 | 3.0 / 2.3 |
| 17 to 22 | 6 × 6 | 3.5 / 2.8 |
| 22 to 30 | 8 × 7 | 4.0 / 3.3 |
| 30 to 38 | 10 × 8 | 5.0 / 3.3 |
| 38 to 44 | 12 × 8 | 5.0 / 3.3 |
| 44 to 50 | 14 × 9 | 5.5 / 3.8 |
| 50 to 58 | 16 × 10 | 6.0 / 4.3 |
Which width tolerance should the keyseat and keyway get?
Choose the fit from how the hub must behave: fixed, sliding along the shaft, or tight with no backlash. In metric work that choice is a pair of ISO tolerance classes on the two grooves; in inch work B17.1 gives width limits by key size and a class of fit.
The key itself is made to a fixed tolerance (class h9 on width in the metric series), and the fit is set entirely by the grooves. The three common metric combinations, with the class limits read from ISO 286-2, are:
- Normal fit (fixed hub): shaft groove N9, hub groove JS9. Against an h9 key both are transition fits: the key usually needs light tapping into the shaft groove, and the hub goes on with little or no play. This is the default for pulleys, sprockets and couplings held by a setscrew or end plate.
- Close fit (no backlash, reversing loads): P9 on both grooves. Use it for reversing or shock torque where any clearance would let the key hammer and wallow the groove.
- Free or sliding fit (hub moves axially): shaft groove H9, hub groove D10, so the hub can travel along a long key.
In the inch series, B17.1 sets keyseat and keyway width as nominal plus a small positive tolerance (in Class 1 it grows with key width), and defines two classes of key fit: Class 1, a clearance or metal-to-metal side fit made with bar-stock keys, and Class 2, a closer side fit that can end up as slight interference. For a stock bar key in a fixed hub, Class 1 is the usual choice; specify Class 2 when the joint sees reversing torque and a key that may need pressing into its seat is acceptable. Whichever system you use, write the fit on the drawing as limits or a tolerance class, never as “to suit key.”
How should keyseat depth be dimensioned on a shaft and hub drawing?
Dimension the shaft keyseat from the bottom of the slot to the opposite side of the shaft, and the hub keyway from the bottom of the slot to the opposite side of the bore. Those two dimensions can be measured directly with a micrometer or caliper, while a depth taken from the curved surface cannot.
A depth called from the top of the shaft has no physical starting point: once the slot is cut, the material it was measured from is gone. That is why inch drawings carry the shaft value as S (bottom of the keyseat to the far side of the shaft) and the hub value as T (bottom of the keyway to the far side of the bore), and why B17.1 tabulates these depth control values. Both account for the chordal height, the small arc of material between the flat at the keyseat edge and the theoretical full circle, which is larger for wide keys on small shafts.
Metric drawings usually show the same idea as d − t1 on the shaft and d + t2 on the hub, with the groove depth tolerance carried through to that combined dimension. A typical callout block on the shaft view reads: keyseat width with its tolerance class (for example 8 N9), the dimension d − t1 with its plus-only limits reversed to minus-only, the keyseat length, and the end form.
Then locate the slot. Width and depth alone do not stop a keyseat from being cut off center or at a slight angle to the axis, and either one makes the key bind in a hub that is also slightly off. Add a symmetry or position tolerance on the slot width relative to the shaft axis (or the bore axis for the hub), with the datum taken from the diameter that actually locates the mating part. ASME Y14.5 removed the symmetry tolerance (term, symbol and concept) in its 2018 edition (summary of the 2018 changes), so on ASME drawings this is normally a position tolerance at RFS or MMC; on ISO drawings symmetry is still in use. If the hub keyway must clock to another feature, such as a setscrew hole or a timing mark, dimension that angle explicitly.
What are the most common keyway drawing mistakes?
The usual errors are a depth with no measurable reference, an ad hoc width tolerance, no location control, an undefined end form, and a coating that changes the slot width. Each one is cheap to fix on the drawing and expensive to find at assembly.
- Depth from the shaft surface. As above, call it from the opposite side of the shaft or bore so it can be measured.
- Ad hoc width tolerance. A generic title-block tolerance on keyseat width is usually several times wider than the named class it replaces (N9 on an 8 mm groove is 0 to −0.036 mm per ISO 286-2): at one end of that band the key rocks, at the other it will not go in. Write the named class (N9, JS9, P9, H9 or D10) or the B17.1 limits, not a generic plus-and-minus value.
- End form not stated. A keyseat cut with an end mill leaves a round-ended (profile) pocket; one cut with a side-milling cutter leaves a long run-out (sled-runner) end. They need different key lengths and give different stress concentrations, so state which one is acceptable and the usable straight length.
- No corner radius. A sharp inside corner at the bottom of the keyseat is a fatigue starter on a shaft in bending. Industry guidance is to call a small bottom radius and chamfer the key edges to clear it.
- Coating ignored. Plating builds on the slot walls as well as the diameter. If the shaft is zinc plated, either mask the keyseat or state that the width limits apply after plating.
- Keyseat running into a shoulder or thread. Leave room for the cutter to exit, or accept the sled-runner end the cutter will leave.
Gauging deserves a line too. A go/no-go width gauge or a gauge block stack checks the slot width, a micrometer or a caliper across the shaft checks the S or d − t1 dimension, and a height gauge over a gauge pin can check location. Saying on the drawing which dimensions are critical helps a supplier choose the gauge before the first part is cut.
What should a keyed shaft or hub RFQ include?
Send the drawing with the key standard and size, the width fit for each groove, the depth dimension from the opposite side, the slot location tolerance, the end form and any coating or masking note. If you supply the key, say so; if the supplier is to provide it, name the key standard and material.
Saola turns shafts, bushings and threaded fittings from bar stock on the CNC turning lathes at its lead CNC plant, and a part that needs off-axis milling is routed to CNC milling at the same plant. Broaching is not among the lead-plant or partner processes listed on this site, so flag any internal hub keyway clearly in the RFQ. Plated shafts go through surface finishing at a partner plant we manage, with the quote naming where each step is done, and that page asks buyers to call out press-fit and other functional surfaces that must stay free of coating; treat a keyseat the same way. Every toleranced dimension on the drawing is measured on the first piece from new tooling and reported, as described on the quality page; ISO 9001 is in progress, not held. Send the package through the quote form, and see the RFQ guide for what else it should carry.
Frequently asked
Is a keyway the same thing as a keyseat?
In ASME usage the slot in the shaft is the keyseat and the slot in the hub is the keyway, and B17.1 is titled “Keys and Keyseats.” In everyday shop language both are called keyways, and metric standards use “keyway” for both grooves, so say “shaft” or “hub” on the drawing to remove any doubt.
Can I use an inch key in a metric shaft?
Only by designing the grooves for that key. The two series do not share widths, so an inch key in a groove cut to a DIN width will either rock or not fit. Pick one system for the shaft, the hub and the key, and state it on every drawing in the assembly.
Where do Woodruff keys fit in?
A Woodruff key is a half-disc key in a curved keyseat, covered in the inch series by ASME B17.2 rather than B17.1. It self-aligns and suits tapered shaft ends and light torque, but the deep seat weakens a small shaft more than a parallel keyseat does.
Should I tolerance the keyseat length tightly?
Usually not. Length only needs to give the key enough bearing area and stay clear of shoulders and grooves. A plus-only tolerance on the usable straight length is normally enough; spend the tight tolerances on width, depth and location.
Sources
- ASME B17.1-1967, Keys and Keyseats (scope), ASME, stabilized 2023, accessed October 2026
- ASME B17.2-1967, Woodruff Keys and Keyseats (scope), ASME, stabilized 2023, accessed October 2026
- DIN 6885-1:2021-11, Drive type fastenings without taper action, parallel keys, keyways, deep pattern, Part 1: Dimensions, tolerances, mass, DIN Media, 2021
- ISO/R 773:1969, Rectangular or square parallel keys and their corresponding keyways (withdrawn), listing by the Swedish Institute for Standards, 1969, accessed October 2026
- ISO 286-2:2010, Geometrical product specifications (GPS), ISO code system for tolerances on linear sizes, Part 2: Tables of standard tolerance classes and limit deviations for holes and shafts, ISO, 2010
- ASME Y14.5-2018, Dimensioning and Tolerancing, ASME, 2018 (reaffirmed 2024)
- GD&T and the new ASME Y14.5-2018 (A-8.4: concentricity and symmetry tolerances removed), Mitutoyo America, 2019