Second moment of area converter
Convert the I value of a beam section between metric and imperial fourth powers.
That's 2,037× as stiff as a 10 mm steel rod, while a timber stud is 2.2× stiffer. A solid square beam about 58.86 mm a side would be just as stiff.
What it means
How stiff is that section?
This is the area moment, about a stated axis. It is not the mass moment of inertia, which has units of kg·m² and belongs to spinning things.
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Three things worth knowing
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1 in⁴ = 41.62 cm⁴
Or 416,231 mm⁴, since 25.4⁴ is a large number. Steel tables in cm⁴ and US tables in in⁴ look wildly different for the same beam.
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Depth counts four times
For a rectangle, I = b·d³/12. Double the depth and the stiffness goes up eightfold; double the width and it only doubles. That single fact shapes every joist and girder.
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Two different inertias
Second moment of area (mm⁴) resists bending; mass moment of inertia (kg·m²) resists spinning. Both are called "I" and they are not interchangeable.
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Background
About the second moment of area
How much a beam bends depends on its material and on the shape of its cross section. The second moment of area, written I, is the shape half: put more material far from the middle and I grows fast, which is why an I-beam has flanges.
The fourth power makes the units awkward: 1 in⁴ is 416,231 mm⁴.