I-Beam Moment of Inertia Calculator
Ix, Iy, area, section modulus and radius of gyration for a doubly-symmetric I-section.
INPUTS
mm
mm
mm
mm
RESULTS
Strong axis Ix
79.99 ×10⁶ mm⁴
Weak axis Iy
6.03 ×10⁶ mm⁴
Area A
5,188 mm²
Section modulus Sx = 2Ix/h
533.3 ×10³ mm³
Radius of gyration rx
124.2 mm
Flanges' share of Ix
84 %
How it works
An I-section is computed as a big rectangle minus the two notches: Ix = [bf·h³ − (bf − tw)·hw³]/12. The flanges, sitting at maximum distance from the neutral axis, contribute most of the stiffness while the thin web mainly carries shear. That division of labour is why the I-beam is the default shape of steel construction.
Worked example
IPE 300 dimensions (h = 300, bf = 150, tf = 10.7, tw = 7.1 mm): Ix = (150·300³ − 142.9·278.6³)/12 ≈ 80×10⁶ mm⁴ (tables give 83.6 with fillets). Around 75–80% of Ix comes from the two flanges alone.
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