Shaft Diameter Calculator
Find the minimum solid shaft diameter required to transmit a given torque, based on allowable shear stress in torsion. Enter torque directly, or calculate it from power and RPM.
Enter your values
Choose input mode, then fill in the fields below.
How this calculator works
For a solid circular shaft carrying pure torque, the minimum diameter that keeps shear stress within an allowable limit is found from the torsion equation:
Where T is the transmitted torque in N·m and τallow is the allowable shear stress in Pa (the calculator converts your MPa entry automatically). If you enter a safety factor, it divides the allowable stress before solving for diameter, so the result already includes that margin.
Getting torque from power and RPM
If you know transmitted power instead of torque, the calculator first converts it:
P is power in watts, N is rotational speed in RPM. The resulting torque is then used in the diameter formula above.
Typical allowable shear stress by material
| Material | Typical allowable shear stress |
|---|---|
| Mild steel (structural) | 40 – 55 MPa |
| Medium carbon steel (AISI 1045) | 55 – 70 MPa |
| Alloy steel (heat treated) | 70 – 100 MPa |
| Stainless steel 304 | 35 – 50 MPa |
These are general reference ranges, not design values — always confirm against your material's datasheet and the applicable design code.
Frequently asked questions
What safety factor should I use?
A factor of 2–4 on yield or endurance strength is typical for general power transmission shafts. Use higher values for shock loading, fatigue, or safety-critical applications.
Does this account for bending, only torsion?
This calculator sizes for torsional shear only. If your shaft also carries significant bending moment, it should be checked against a combined-stress (equivalent torque) criterion, which typically gives a larger diameter.