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Keyway Size Calculator

Work out keyway size instantly with clear inputs, formula shown and shareable results.

About Keyway Size Calculator

The key carries torque as a tangential force F = 2T/d at the shaft surface. Shearing across the key gives required width as F / (allowable shear x length). Standard proportions take width as a quarter of the shaft diameter and height as a sixth, so the check is usually whether the standard key is long enough rather than wide enough.

Formulas

Tangential force

F = 2T / d

Key shear

Width required = F / (allowable shear x key length); standard width = d / 4

Worked example

With the default values below, Keyway Size Calculator returns Key width required of 4.17. Change any input above and every figure here is recalculated in your browser.

Inputs used

Shaft diameterminimum 5 mm
40 mm
Transmitted torqueminimum 1 N·m
300 N·m
Key lengthminimum 5 mm
60 mm
Allowable shear stressminimum 5 MPa
60 MPa

Results

Key width required
4.17 mm
Standard key height
6.67 mm
Tangential force on key
15,000 N
Shear stress at standard width
25.00 MPa

Step by step

  1. 1Values used
  2. 2Tangential force
  3. 3Key shear
  4. 4Key width required= 4.17 mm
  5. 5Standard key height= 6.67 mm
  6. 6Tangential force on key= 15,000 N
  7. 7Shear stress at standard width= 25.00 MPa

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Preliminary estimate only. Key and keyway design must be verified against the relevant dimensional standard and checked by a qualified engineer for both shear and bearing stress.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Key and keyway design must be verified against the relevant dimensional standard and checked by a qualified engineer for both shear and bearing stress.

Frequently Asked Questions

How is the Keyway Size figure worked out?

F = 2T / d. The key carries torque as a tangential force F = 2T/d at the shaft surface.

Should crushing also be checked?

Yes. Bearing stress on the key side, F divided by half the height times the length, often governs before shear does, especially with short keys in soft hubs.

What values do I need to enter?

This calculator takes 4 inputs: Shaft diameter, Transmitted torque, Key length, Allowable shear stress. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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