Quick reference
Common moment of inertia conversions
The fourth-power factor makes inertia conversions much larger than ordinary length or area conversions.
| From | Equivalent |
|---|---|
| 1 in⁴ | 416,231.426 mm⁴ |
| 1 cm⁴ | 10,000 mm⁴ |
| 1 ft⁴ | 20,736 in⁴ |
| 1 m⁴ | 10¹² mm⁴ |
Section property utility
Moment of Inertia Unit Converter
Convert second moment of area (area moment of inertia) between mm⁴, cm⁴, m⁴, in⁴, and ft⁴ for beam deflection and stress calculations.
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Convert inertia units
Next step
Open Section Properties
Calculate a complete custom section and send its area, inertia, and extreme-fibre distances into the Beam Calculator.
Engineering context
Moment of inertia in beam analysis
Second moment of area, usually written I, measures how cross-sectional area is distributed about an axis. It directly affects elastic beam stiffness EI.
Because I uses length to the fourth power, even a small unit mismatch creates a very large error in deflection or bending stress results.
Use the correct section axis
Ix and Iy are generally different. Convert the value about the bending axis used in the beam model.
Where this conversion is used
Structural engineering applications
- Beam deflection stiffness EI
- Bending stress calculations
- Strong- and weak-axis checks
- Custom section imports
Questions engineers ask
Inertia conversion FAQ
How many mm⁴ are in one in⁴?
One in⁴ is approximately 416,231.426 mm⁴.
Why are moment of inertia units to the fourth power?
Second moment of area integrates squared distance over an area, resulting in a fourth-power length dimension.
Is mass moment of inertia the same thing?
No. This page converts second moment of area used in beam bending, not rotational mass moment of inertia.
Conversion factors follow SI and U.S. customary unit relationships. Rounded reference values are for convenience; the live converter uses higher-precision factors.