Shaft Size Calculator
Enter the power in kW or hp, the shaft speed in rpm and the allowable shear stress. The calculator works out the torque, the shaft diameter needed for torsion and the next standard size.
- Shaft diameter
- Torque
- Standard size
40 MPa suits plain mild steel shafts with a keyway. Use a higher value only for stronger steels.
Shaft size follows the torque it carries. Torque = power × 9,549 ÷ rpm. A 7.5 kW motor at 1,450 rpm makes about 49 N·m. At 40 MPa allowable shear, d = ∛(16T ÷ πτ) is about 18.5 mm, so a standard 20 mm shaft.
Shaft size chart
1,450 rpm, 40 MPa. Search for a power.
| Power (kW) | Torque (N·m) | Diameter (mm) | Standard (mm) |
|---|---|---|---|
| 0.75 | 4.9 | 8.6 | 10 |
| 1.5 | 9.9 | 10.8 | 12 |
| 3 | 19.8 | 13.6 | 15 |
| 5.5 | 36.2 | 16.6 | 20 |
| 7.5 | 49.4 | 18.5 | 20 |
| 11 | 72.4 | 21 | 25 |
| 15 | 98.8 | 23.3 | 25 |
| 22 | 144.9 | 26.4 | 30 |
| 37 | 243.7 | 31.4 | 35 |
| 55 | 362.2 | 35.9 | 40 |
| 75 | 493.9 | 39.8 | 40 |
Solid steel shaft.
Power and shaft
1,450 rpm. Slide the power.
The shaft is drawn with its diameter.
How to size a shaft
Power to diameter. Step through it.
Power
kW.
Slower shafts carry more torque for the same power, so they must be thicker.
Torsion only or with bending
Shaft loads. Compare them.
Pure twist.
Side loads.
Span.
Gears and belt pulleys pull sideways on a shaft as well as twisting it.
Before you build
Shaft checks. Tick each item.
Tick each item as you go.
Fit bearings close to pulleys and gears to keep bending small.
What you get
Shaft sizing. Each tile explains one result.
mm.
N·m.
Size.
At standard.
Converted.
Shaft diameter grows with the cube root of torque.
Frequently Asked Questions
No question matches that search.
How do I calculate shaft diameter?
Find the torque, then use the diameter equals the cube root of sixteen times torque over pi times allowable shear stress.
How do I convert kilowatts to torque?
Torque in newton metres equals kilowatts times nine thousand five hundred and forty-nine divided by rpm.
What shear stress should I use?
About forty megapascals for plain mild steel shafts with a keyway.
Does speed change shaft size?
Yes. Slower shafts carry more torque for the same power.
Why add margin for a keyway?
The keyway weakens the shaft by about a quarter.
Is this enough for a pulley shaft?
No. Pulleys add bending, which needs a combined check.
What standard shaft sizes are there?
Common metric sizes include twenty, twenty-five, thirty and forty millimetres.
Strong enough to turn
Size the shaft from its power and speed, and it carries the torque without twisting or failing.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.