What is the basic formula for shaft diameter from torque?

d³ = (16 × T × FS) / (π × τ_allow) where T = torque (Nm), FS = safety factor, τ_allow = allowable shear stress (MPa).

What is a typical safety factor for shafts?

2–3 for steady loads, 3–4 for shock or reversing loads, 4–6 for critical applications.

Mild steel: 40–55 MPa, C45: 80–100 MPa, Alloy steel (42CrMo4): 120–150 MPa.

Calculate minimum shaft diameter from transmitted torque, material strength, and safety factor. Fast and accurate mechanical design tool.

Required Shaft Diameter

Ø 42.8 mm

From 500 Nm torque, C45 steel, FS = 3.0

Featured: Pure Torsion (ASME), Common Materials, Safety Factor

Torque T (Nm)

OR Power P (kW)

Speed N (RPM) Used only if power is entered

Material

Factor of Safety (FS)

Pure Torsion (ASME / Standard)

Most common method for shafts under pure torsional load.

d = [ (16 × T × FS) / (π × τ_allow) ]^(1/3)

Enter parameters and click "Calculate Diameter" to see results

Standard formula used worldwide for shafts transmitting power without significant bending.

Accounts for shock loads, fatigue, material variation, and keyway stress concentration.

Predefined allowable shear stresses based on common engineering steels.

Mild Steel: 40–55 MPa (conservative)

C45 / EN8: 80–100 MPa

42CrMo4 / EN19: 120–150 MPa

Factor of Safety Guide:

• Steady load: 2.0–2.5

• Minor shock: 3.0–4.0

• Heavy shock/reversing: 4.0–6.0