What size key does a 1 inch shaft need?

A 1/4 in square key, seated 1/8 in deep in the shaft and 1/8 in deep in the hub. The 1 in shaft falls in the 15/16 to 1-1/4 in range in ASME B17.1. In metric practice the equivalent is a 8 × 7 mm key to DIN 6885 for shafts of 22–30 mm, which is not a square key — metric practice favours rectangular keys at this size.

Why is a key designed to shear?

Because a sheared key is cheap to replace and a twisted shaft is not. The key is deliberately the weakest element in the drive train: sized from the shaft diameter rather than from the torque, it will fail before the shaft yields under overload. That is also why keyways are kept short — every extra millimetre of keyseat weakens the shaft at the point where the stress concentration already is.

How much does a keyway weaken a shaft?

Roughly 10 to 20% of torsional strength, from two effects acting together: the material removed, which for a 1 in shaft with a 1/4 in key is 1/8 in of depth, and the sharp internal corner, which acts as a stress concentration. Slightly rounded corners and a shorter keyseat both help. Where the shaft is strength-critical, a spline or a shrink disc avoids the problem entirely.

What is the difference between a square and a rectangular key?

A square key has equal width and height and sits half into the shaft and half into the hub. A rectangular key is taller than wide, typically in a ratio around 1.25 to 1, and sits less deeply in the shaft and more deeply in the hub. The rectangular form carries more torque in the same keyseat width because the bearing area on the hub side is larger — useful when the shaft is the limiting component.

When would I use a Woodruff key instead?

For small shafts, for location rather than power transmission, and where the hub must be adjustable along the shaft. A Woodruff key sits in a semicircular seat milled into the shaft, so it stays in place during assembly and the hub can slide over it. Its torque capacity is lower than a square key of the same width because the seat is shallower, so it is not used for heavy drives.

Keyway Sizes

Square key and keyseat dimensions for shafts from 1/4 in to 5-1/2 in, with shaft and hub keyseat depths and the DIN 6885 metric equivalent for each range.

Data verified 2026-09-29 · based on ASME B17.1-1967 (R2017)

Quick Answer

For a square key, the keyseat depth is half the key width in both the shaft and the hub — a 1/4 in key sits 1/8 in deep in each. Key width is selected from the shaft diameter: a 1 in shaft takes a 1/4 in key, and a 2 in shaft takes a 1/2 in key. The key is deliberately the sacrificial part — on overload it shears and protects the shaft.

Find a shaft size

Square Key and Keyseat Dimensions (ASME B17.1)

Shaft diameter in^[1]	Key width × height^[1]	Key width in^[2]	Keyseat depth — shaft in^[2]	Keyseat depth — hub in^[2]	Key width mm^[2]	Keyseat depth mm^[2]	Metric equivalent^[3]
1/4 – 5/16 #	3/32	0.0938	0.0469	0.0469	2.38	1.19	DIN 6885: 3 × 3 mm for shafts 8–10 mm
3/8 – 7/16 #	1/8	0.1250	0.0625	0.0625	3.17	1.59	DIN 6885: 4 × 4 mm for shafts 10–12 mm
1/2 – 9/16 #	1/8	0.1250	0.0625	0.0625	3.17	1.59	DIN 6885: 5 × 5 mm for shafts 12–17 mm
5/8 – 7/8 #	3/16	0.1875	0.0938	0.0938	4.76	2.38	DIN 6885: 6 × 6 mm for shafts 17–22 mm
15/16 – 1-1/4 #	1/4	0.2500	0.1250	0.1250	6.35	3.17	DIN 6885: 8 × 7 mm for shafts 22–30 mm
1-5/16 – 1-3/8 #	5/16	0.3125	0.1562	0.1562	7.94	3.97	DIN 6885: 10 × 8 mm for shafts 30–38 mm
1-7/16 – 1-3/4 #	3/8	0.3750	0.1875	0.1875	9.52	4.76	DIN 6885: 12 × 8 mm for shafts 38–44 mm
1-13/16 – 2-1/4 #	1/2	0.5000	0.2500	0.2500	12.70	6.35	DIN 6885: 14 × 9 mm for shafts 44–50 mm
2-5/16 – 2-3/4 #	5/8	0.6250	0.3125	0.3125	15.88	7.94	DIN 6885: 16 × 10 mm for shafts 50–58 mm
2-13/16 – 3-1/4 #	3/4	0.7500	0.3750	0.3750	19.05	9.52	DIN 6885: 20 × 12 mm for shafts 58–65 mm
3-5/16 – 3-3/4 #	7/8	0.8750	0.4375	0.4375	22.22	11.11	DIN 6885: 22 × 14 mm for shafts 65–75 mm
3-13/16 – 4-1/2 #	1	1.0000	0.5000	0.5000	25.40	12.70	DIN 6885: 25 × 14 mm for shafts 75–85 mm
4-9/16 – 5-1/2 #	1-1/4	1.2500	0.6250	0.6250	31.75	15.88	DIN 6885: 32 × 18 mm for shafts 95–110 mm

Dimensions are for square keys, where key width equals key height. Keyseat depth is half the key width in the shaft and half in the hub, so the key is captured equally by both — the depth columns are therefore derived, not transcribed, and are checked against the half-width relationship on every row. Rectangular keys (key height greater than width) split differently and are a separate table in the standard. Pratt & Whitney and Woodruff keys are also separate series with their own dimensions. Values are nominal minima; the standard also specifies a keyseat width tolerance that depends on whether the fit is to be tight or loose.

How Key Size Is Selected

The key is sized from the shaft diameter, not from the torque. That surprises people who expect a heavily loaded shaft to need a bigger key, but the reasoning is sound: a key proportioned to the shaft will shear before the shaft yields, so the key acts as a mechanical fuse. Sizing the key for the torque instead would remove that protection and risk a broken shaft rather than a sheared key.

The proportions in the table are what that logic produces. A 1 in shaft takes a 1/4 in key, so key width is a quarter of shaft diameter, and the keyseat removes 1/8 in of material from the shaft — about 12% of its diameter, concentrated at one point on the circumference. The shaft is weaker everywhere the keyseat is cut, which is why keyways are kept as short as the torque requires and why a shaft with a long keyway is not interchangeable with a plain one for strength purposes.

Deeper shafting practice: a keyway reduces the shaft's torsional strength by roughly 10 to 20% depending on the depth-to-diameter ratio, and it introduces a stress concentration. Where a shaft is strength-critical, the alternatives are a spline (which distributes the load over many teeth), a taper with a friction fit, or a shrink disc — all of which avoid a single sharp-cornered keyseat.

Square, Rectangular and Woodruff Keys

Three key types cover almost all practice, and they are not interchangeable.

Square keys have equal width and height, and are set half into the shaft and half into the hub. This is the table above, and it is the default for general machinery. Because the key is captured equally, the shaft and hub contribute equally to the drive.

Rectangular keys are taller than they are wide — typically by a ratio of about 1.25 to 1. They carry more torque in the same keyseat width because the greater height increases the bearing area on the hub side, which is where the load is usually highest. The keyseat depths are then asymmetric: the shaft takes less than half the height and the hub takes more. Use them when the hub wall is thick enough and the shaft is strength-limited.

Woodruff keys are half-moon shaped and sit in a milled semicircular seat in the shaft, with the flat side engaging the hub. They are used for small shafts, for location rather than power transmission, and in applications where the key must allow the hub to be adjusted along the shaft. They are dimensioned by the American Woodruff Key series, entirely separately from this table.

Fits matter as much as dimensions. A key that is loose in the keyseat will hammer and fail in fatigue; one that is tight will not assemble. The standard specifies keyseat width tolerances for both a tight and a loose fit, and the choice between them is a design decision, not a manufacturing convenience.

Frequently Asked Questions

What size key does a 1 inch shaft need?: A 1/4 in square key, seated 1/8 in deep in the shaft and 1/8 in deep in the hub. The 1 in shaft falls in the 15/16 to 1-1/4 in range in ASME B17.1. In metric practice the equivalent is a 8 × 7 mm key to DIN 6885 for shafts of 22–30 mm, which is not a square key — metric practice favours rectangular keys at this size.
Why is a key designed to shear?: Because a sheared key is cheap to replace and a twisted shaft is not. The key is deliberately the weakest element in the drive train: sized from the shaft diameter rather than from the torque, it will fail before the shaft yields under overload. That is also why keyways are kept short — every extra millimetre of keyseat weakens the shaft at the point where the stress concentration already is.
How much does a keyway weaken a shaft?: Roughly 10 to 20% of torsional strength, from two effects acting together: the material removed, which for a 1 in shaft with a 1/4 in key is 1/8 in of depth, and the sharp internal corner, which acts as a stress concentration. Slightly rounded corners and a shorter keyseat both help. Where the shaft is strength-critical, a spline or a shrink disc avoids the problem entirely.
What is the difference between a square and a rectangular key?: A square key has equal width and height and sits half into the shaft and half into the hub. A rectangular key is taller than wide, typically in a ratio around 1.25 to 1, and sits less deeply in the shaft and more deeply in the hub. The rectangular form carries more torque in the same keyseat width because the bearing area on the hub side is larger — useful when the shaft is the limiting component.
When would I use a Woodruff key instead?: For small shafts, for location rather than power transmission, and where the hub must be adjustable along the shaft. A Woodruff key sits in a semicircular seat milled into the shaft, so it stays in place during assembly and the hub can slide over it. Its torque capacity is lower than a square key of the same width because the seat is shallower, so it is not used for heavy drives.

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

#	Source	Type	Revision / method
[1]	ASME B17.1 — Keys and Keyseats	standard	ASME B17.1-1967 (R2017)
[2]	Value computed from the standard's defining relationship	derived	Computed at build time from the defining formula stated on the page and verified against every row, then checked against known standard values held as anchors. Nothing in this column was transcribed.
[3]	DIN 6885-1 — Drive type fastenings without taper action, parallel keys and keyways	standard	DIN 6885-1:1968

Data Sources

Standard	Revision	What it covers on this page
ASME B17.1 — Keys and Keyseats	ASME B17.1-1967 (R2017)	the key sizes, the shaft diameter ranges and the keyseat depths
DIN 6885-1 — Parallel keys and keyways	DIN 6885-1:1968	the metric key equivalents in the last column
ISO 773 / ISO 2491 — Rectangular and thin parallel keys	ISO 773:2017	the rectangular key series referenced in the discussion

Cross-checked against:

Key size against shaft diameter checked against the ASME B17.1 selection table
Keyseat depths verified at build time against the half-width relationship for square keys on every row
Metric equivalents checked against the DIN 6885-1 shaft diameter ranges rather than converted arithmetically
Four values held as known-value checks, including the key width for a 1 in shaft

Derived values — the following values on this page are calculated, not taken directly from the standard:

Value	How it is derived
Key width, keyseat depths and metric conversions	Key width in inches from the fractional value. Both keyseat depths are the key width × 0.5, which is the defining property of a square key and is verified at build time on every row. Millimetre values are inches × 25.4.

Dimensions are nominal and apply to square keys to ASME B17.1. Keyseat width tolerances depend on whether a tight or loose fit is specified and are not shown here — take them from the standard. Rectangular, Woodruff and Pratt & Whitney keys have their own dimensional series. A keyseat reduces shaft torsional strength by roughly 10–20%, so verify the shaft separately where the drive is strength-critical.

Accuracy and use. The values on this page are compiled from the published standards and cross-checked sources listed above. Where values are derived, the derivation is stated. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

Cite This Page

Every value on this page is traceable to the sources listed above. If you use the data in a document, paper or report, cite it as:

Plain text

Engineering Reference. "Keyway Sizes." https://refmanual.com/charts/keyway-sizes/ (accessed 2026-09-29). Data from ASME B17.1 — Keys and Keyseats, ASME B17.1-1967 (R2017).

BibTeX

@misc{keyway-sizes,
  title        = {Keyway Sizes},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/charts/keyway-sizes/}},
  note         = {Accessed: 2026-09-29},
  source       = {ASME B17.1 — Keys and Keyseats, ASME B17.1-1967 (R2017)}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - Keyway Sizes
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/charts/keyway-sizes/
PY  - 2026
DA  - 2026-09-29
N1  - Source: ASME B17.1 — Keys and Keyseats, ASME B17.1-1967 (R2017)
ER  -

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