Work out shaft diameter instantly with clear inputs, formula shown and shareable results.
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.
Torque
T lb·in = 63025 x hp / rpm
Diameter in torsion
d = (16 T / (pi x allowable shear))^(1/3)
Preliminary estimate only. Shaft design must be verified by a qualified engineer for combined bending and torsion, fatigue, keyway stress concentration and deflection.
T lb·in = 63025 x hp / rpm. Torque in pound-inches is 63,025 times horsepower divided by rpm.
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.
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.