Skip to content
ENExplore Calculators

Shaft Diameter Calculator

Work out shaft diameter instantly with clear inputs, formula shown and shareable results.

About Shaft Diameter Calculator

Torque in pound-inches is 63,025 times horsepower divided by rpm. For a solid circular shaft in pure torsion the maximum shear stress is 16T / (pi d³), so inverting gives the diameter needed for a chosen allowable stress. Bending from belt or gear loads must be superimposed separately.

Formulas

Torque

T lb·in = 63025 x hp / rpm

Diameter in torsion

d = (16 T / (pi x allowable shear))^(1/3)

Worked example

With the default values below, Shaft Diameter Calculator returns Shaft diameter required of 0.674. Change any input above and every figure here is recalculated in your browser.

Inputs used

Transmitted powerminimum 0.1 hp
10 hp
Shaft speedminimum 1 rpm
1,750 rpm
Allowable shear stressminimum 100 psi
6,000 psi

Results

Shaft diameter required
0.674 in
Design torque
360.1 lb·in
Polar section modulus needed
0.0600 in³
Next standard size
0.750 in

Step by step

  1. 1Values used
  2. 2Torque
  3. 3Diameter in torsion
  4. 4Shaft diameter required= 0.674 in
  5. 5Design torque= 360.1 lb·in
  6. 6Polar section modulus needed= 0.0600 in³
  7. 7Next standard size= 0.750 in

Assumptions and limits

  • The result is derived only from the 3 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 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Preliminary estimate only. Shaft design must be verified by a qualified engineer for combined bending and torsion, fatigue, keyway stress concentration and deflection.

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

Preliminary estimate only. Shaft design must be verified by a qualified engineer for combined bending and torsion, fatigue, keyway stress concentration and deflection.

Frequently Asked Questions

How is the Shaft Diameter figure worked out?

T lb·in = 63025 x hp / rpm. Torque in pound-inches is 63,025 times horsepower divided by rpm.

Is torsion alone enough?

Rarely. Overhung belt or gear loads add bending, and keyways cut into the section. Combined stress and a stress concentration factor should both be checked.

What values do I need to enter?

This calculator takes 3 inputs: Transmitted power, Shaft speed, Allowable shear stress. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

Related Calculators

Shaft Diameter Calculator | Calcrivo