What is the difference between a keyway and a keyseat?

A keyseat is the slot cut into the shaft, and a keyway is the matching slot cut into the hub, bore, or gear. Together they form the mating pair. In common usage the terms are often used interchangeably.

Which standard should I use — ASME B17.1 or DIN 6885?

Use the standard and edition required by the controlled drawing. ASME B17.1-1967 (S2023) is the current reaffirmed inch reference listed by ANSI. DIN Media lists DIN 6885-1:2021-11 as the current metric deep-pattern edition. The metric table in this free lookup is explicitly a legacy 1968 geometry reference, not a reproduction of the licensed 2021 edition.

What are the keyway width tolerances for a normal, close or free fit?

For metric parallel keys the key width is h9. A normal fit uses an N9 shaft keyseat and a JS9 hub keyway, a close fit uses P9 for both, and a free (sliding) fit uses H9 on the shaft and D10 in the hub. For a 12 mm key that is key 0/−0.043, N9 0/−0.043, JS9 ±0.0215, P9 −0.018/−0.061, H9 +0.043/0 and D10 +0.120/+0.050 mm. Depth tolerance on t1 and t2 is +0.1 mm up to b = 6, +0.2 up to b = 28 and +0.3 above.

How deep should a keyway be cut in a 1 inch shaft?

A 1 in shaft takes a 1/4 × 1/4 in square key. The keyseat is H/2 = 0.125 in deep at its side wall, but the shaft crown is higher by the chordal height Y = (1 − √(1 − 0.25²))/2 = 0.0159 in, so the cutter goes 0.1409 in below the shaft surface. Check it as S = 1 − 0.0159 − 0.125 = 0.859 in across the shaft; the matching hub dimension is T = 1 − 0.0159 + 0.125 + 0.005 = 1.114 in across the bore.

What is the chordal height of a keyseat?

It is the height of the arc the keyseat removes above its side walls: Y = (D − √(D² − W²)) / 2 for shaft diameter D and key width W. Standard depths (H/2, t1, t2) are measured at the side wall, so add Y when you touch off the cutter on top of the shaft. For a 40 mm shaft with a 12 mm key, Y = 0.92 mm, so a 5.0 mm t1 needs a 5.92 mm cut from the crown.

How should a shaft keyway be dimensioned on a drawing?

Give the keyseat width with its fit, the keyseat length, and the depth. Depth is usually dimensioned from the bottom of the keyseat to the opposite side of the shaft (S, or d − t1 on metric drawings) because it can be checked with a micrometer, and the hub keyway from its top to the opposite side of the bore (T, or d + t2). A 40 mm shaft with a 12 × 8 key is dimensioned 35.0 mm on the shaft and 43.3 mm in the hub.

What key size fits a 12 mm shaft?

In the legacy DIN 6885-1 rows this tool uses, a shaft over 10 mm up to and including 12 mm takes a 4 × 4 mm key with a 2.5 mm shaft keyseat depth. Above 12 mm, up to 17 mm, the row changes to a 5 × 5 mm key with 3.0 mm depth.

My shaft diameter is exactly on a range boundary — which row applies?

Each displayed interval is lower-exclusive and upper-inclusive: min < d ≤ max. For example, 8.000 mm moves to the row ending at 8 mm, while any value greater than 8 mm moves to the next row.

Does this cover Woodruff (semi-circular) keys?

No. This tool covers parallel rectangular keys per ANSI B17.1 and DIN 6885. Woodruff key dimensions are defined in a separate standard (ANSI B17.2 / DIN 6888) and require a different calculation.

Can I use these dimensions directly for CNC programming?

No. Treat the output as a preliminary lookup. Confirm the current standard edition, selected fit, numeric deviations, corner radii, key length, material and drawing requirements before creating a CNC program or releasing a part.

Keyway Size Calculator

Enter a shaft diameter to get the key size, shaft keyseat depth and hub keyway depth, plus the S and T (or d − t1 and d + t2) dimensions you check with calipers. Inch rows follow ASME B17.1 (square or rectangular keys, shafts 5/16 to 30 in); metric rows reproduce legacy DIN 6885-1:1968 (6 to 500 mm) with ISO 286 width fits.

Pick the key from the shaft diameter range, then cut the shaft keyseat to half the key height (ASME B17.1) or to t1 (DIN 6885), and the hub keyway to H/2 + 0.005 in or t2.

Shaft Input

Standard

Shaft Diameter (inches)

Key section

Quick examples

Enter a shaft diameter on the left to see standard keyway dimensions.

Copied!

Quick answer

Pick the key from the shaft diameter range, then cut the shaft keyseat to half the key height (ASME B17.1) or to t1 (DIN 6885), and the hub keyway to H/2 + 0.005 in or t2. Depths are measured at the side wall; because the shaft is round, the cut from the shaft crown is deeper by the chordal height.

Examples

Shaft diameter	Key W × H	Shaft depth	Hub depth
3/4 in	3/16 × 3/16 in	0.0938 in	0.0988 in
1 in	1/4 × 1/4 in	0.1250 in	0.1300 in
1-1/2 in	3/8 × 3/8 in	0.1875 in	0.1925 in
12 mm	4 × 4 mm	2.5 mm	1.8 mm
40 mm	12 × 8 mm	5.0 mm	3.3 mm

Enter your shaft diameter for key size, shaft and hub depths, S/T dimensions and fit tolerances.

Summary

A keyseat is a slot cut into a shaft and a matching keyway is cut into a hub or gear. This calculator maps a shaft diameter to the standard parallel-key row, then works out the depths a machinist needs: shaft and hub depth at the side wall, the chordal height Y = (D − √(D² − W²))/2, the total cutter depth from the shaft surface, and the caliper dimensions across the shaft (S or d − t1) and across the bore (T or d + t2). Metric results add ISO 286 width deviations for normal, close and free fits. It is a reference aid, not a substitute for the licensed current standard, the controlled drawing, torque sizing or inspection.

How it works

Enter the shaft diameter in either inches (ANSI) or millimeters (DIN/metric).
Select the applicable standard: ANSI B17.1 or DIN 6885.
The tool selects the table interval that actually contains the entered diameter; it does not round to a nearby shaft size.
Key width and height, shaft and hub depths, chordal height, cutter depth and the S/T (d − t1, d + t2) dimensions are calculated.
Inch results use the ASME B17.1 depth-control formulas S = D − Y − H/2 and T = D − Y + H/2 + 0.005; metric results add ISO 286 width deviations for the chosen fit.
Copy any value to clipboard with one click for use in drawings or CNC programs.

Use cases

Selecting keyway dimensions during mechanical shaft design.
Verifying dimensions on a machinist or engineering drawing.
Specifying key stock size when ordering hardware.
Cross-checking ANSI vs. DIN equivalents on international projects.
Teaching students standard shaft-key fitting principles.
Quick field reference on a tablet or phone without paper charts.

Frequently Asked Questions

Last updated: 2026-10-09 · Reviewed by Nham Vu

Keyway Size Calculator

Shaft Input

Dimensions

Fits and depth tolerances

Quick answer

Examples

Summary

How it works

Use cases

Frequently Asked Questions

Keyway Size Calculator

Use the Keyway Size Calculator

Shaft Input

Dimensions

Fits and depth tolerances

Quick answer

Examples

Summary

How it works

Use cases

Frequently Asked Questions