Enter any two of the inner diameter, outer diameter, and diameter ratio into the calculator to determine the missing value of the hollow shaft.

Hollow Shaft Diameter Calculator

Enter any 2 values to calculate the missing variable

Related Calculators

Hollow Shaft Diameter and Ratio Guide

Use this calculator when you know any two of the three core values: inner diameter, outer diameter, or diameter ratio. Enter both diameters in the same unit, then solve for the missing value; the ratio itself is unitless. ([calculator.academy](https://calculator.academy/hollow-shaft-diameter-calculator/))

Core Formulas

Find Relationship Use Case
Diameter ratio 
HSDR = \frac{ID}{OD}
 Use when both diameters are known.
Inner diameter 
ID = HSDR \cdot OD
 Use when the target ratio and outer diameter are known.
Outer diameter 
OD = \frac{ID}{HSDR}
 Use when the bore size and ratio are known.
Wall thickness 
t = \frac{OD - ID}{2}
 Useful for checking how much material remains in the shaft wall.

The primary relationship above matches the calculator’s hollow shaft diameter ratio equation, and the other forms are direct algebraic rearrangements of that same ratio. ([calculator.academy](https://calculator.academy/hollow-shaft-diameter-calculator/))

Variable Reference

Symbol Meaning Typical Unit
HSDR Hollow shaft diameter ratio Unitless
ID Inner diameter mm, in, or any consistent length unit
OD Outer diameter mm, in, or any consistent length unit
t Wall thickness Same unit as the diameters

Input Checks

Check Why It Matters
Inner diameter must be smaller than outer diameter. Otherwise the geometry is not physically possible.
For a true hollow shaft, the ratio should be greater than zero and less than one. Values near one indicate a thin wall; values near zero indicate a very thick wall.
Use consistent units for both diameters. Mixing millimeters and inches will produce the wrong result.
Convert percentages to decimals before entry. For example, 75% should be entered as 0.75.

How to Interpret the Ratio

Ratio Range Wall Condition General Meaning
Below 0.50 Thicker wall More material in the section and a smaller internal opening.
0.50 to 0.80 Moderate wall A balanced geometry for many general shaft layouts.
Above 0.80 Thin wall Larger internal passage and lower weight, but less wall material.

Example Results

Known Values Solve For Result
Inner diameter = 30 mm, outer diameter = 40 mm Diameter ratio 0.75
Diameter ratio = 0.60, outer diameter = 50 mm Inner diameter 30 mm
Inner diameter = 24 mm, diameter ratio = 0.80 Outer diameter 30 mm

Helpful Notes

  • The ratio only describes the size of the hole relative to the outside diameter; it does not give the absolute shaft size by itself.
  • Once inner and outer diameter are known, wall thickness is usually the next value checked for fit, strength, and manufacturability.
  • If you are comparing multiple designs, the ratio is a quick way to see which option has the larger bore or thicker wall.