What this converter does
This converter changes a moment of inertia — second moment of area — from in⁴ to mm⁴ and back, and also to cm⁴ used in metric section tables. Type a value, pick your units, and read the result instantly.
One in⁴ is exactly 416,231 mm⁴ (25.4⁴), because an inch is exactly 25.4 mm. Swap the arrow to convert the other way.
The units it covers
Second moment of area has units of length⁴, so each unit converts through mm⁴ by a fixed factor.
View all units & their values
| Unit | Symbol | Value | Mainly used |
|---|---|---|---|
| Millimetre⁴ | mm⁴ | 1 | Metric section data |
| Centimetre⁴ | cm⁴ | 10,000 | Steel section tables (Ix) |
| Inch⁴ | in⁴ | 416,231 | US steel section tables |
| Metre⁴ | m⁴ | 1e12 | Large sections, analysis |
The formula
Each length⁴ unit has a fixed value in mm⁴, so the conversion is one multiplication:
result = value × factor_from ÷ factor_toWhere:
- value = the second moment you typed
- factor_from = its value in mm⁴
- factor_to = the target unit’s value in mm⁴
Worked example
Convert 120 in⁴ (a steel beam Ix) to cm⁴.
120 × 416,231 = 4.995e7 mm⁴4.995e7 ÷ 10,000 = 4,995 cm⁴So 120 in⁴ ≈ 5,000 cm⁴ — matching the metric table value.
The units in this example
The US customary unit for moment of inertia, from AISC steel section tables.
- 1 in⁴ = 416,231 mm⁴
- 1 in⁴ = 41.62 cm⁴
- 100 in⁴ ≈ 4,162 cm⁴
- factor = 25.4⁴
The SI-based unit. Metric tables usually give cm⁴ (1 cm⁴ = 10,000 mm⁴) for beam and column sections.
- 1 in⁴ = 416,231 mm⁴
- 1 cm⁴ = 10,000 mm⁴
- 1 m⁴ = 1e12 mm⁴
- 1 mm⁴ ≈ 2.4e-6 in⁴