Shaft Size Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

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The quickest way to trust a shaft size estimate is to see how the answer responds when an input changes. Mechanical design calculations become much more useful when you can see the exact ideal relationship behind the headline number and where real hardware can depart from it. The sections below show what this page calculates, how it does it, and where the simplified model stops.

What this calculator does

The purpose of the Shaft Size Calculator is to evaluate solid shaft diameter from Power transmitted (P), Shaft rotation speed (N), Allowable shear stress (τ). It is most useful for quick comparisons or hand-checks where the input definitions and displayed units remain part of the answer.

How to use it

Fill in the active quantities—Power transmitted (P), Shaft rotation speed (N), Allowable shear stress (τ), Ratio of inner to outer diameter (k)—and leave output-only boxes for the calculator to derive. Pay special attention to signs, angles, and whether a dimension is a radius, diameter, area, or length. Read the result together with its displayed unit.

How the calculation works

If torque is not entered directly, it is derived from power and speed as T = P/ω. For a solid circular shaft the page uses d = [16T/(πτallow)]^(1/3). Hollow-shaft diameter adds the inner/outer ratio through the factor 1−k⁴.

Example

Using the default example on the page (Power transmitted (P) = 10 kW; Shaft rotation speed (N) = 1500 rpm; Allowable shear stress (τ) = 40 MPa; Ratio of inner to outer diameter (k) = 0.6), the calculator returns solid shaft diameter of 0.020088 m. If a small input change sends the value in the opposite direction from the equation, recheck the selected unit and sign convention.

How to interpret the result

Read the solid shaft diameter as an ideal mechanical estimate for the entered geometry, speed, load, ratio, or material property. Keep the displayed unit attached and use any supporting values to confirm that ratios and directions are internally consistent.

Limitations and notes

The visible design-basis selector does not alter the active path; torque is taken directly when present or inferred from power and RPM. The diameter formulas cover pure torsion with allowable shear stress and do not include bending, keyways, stress concentration, fatigue, shock, critical speed, or code safety factors.