An H-beam is composed of three sections. Two parallel flanges form the ends of the beam, and a stretch of metal, the beam's webbing, runs between them. The lengths of these sections can withstand compressive forces, letting the H-beam bear a significant load without bending. The beam's size describes its overall resistance to bending forces. This value, the beam's area moment of inertia, is the product of the beam's dimensions, and it takes the unit of length raised to the power of 4.
Raise the length of each of the H-beam's flanges to the power of 3. For example, if each flange is 6 inches long: 6^3 = 216 in^3.
Multiply this answer by the width of a flange. For example, if each flange is 2 inches thick: 216 × 2 = 512 in^4.
Double this answer because the beam has two flanges: 512 × 2 = 1,024 in^4.
Repeat Steps 1 to 3 with the webbing between the flanges. For example, if the webbing is 6.5 inches long and 2.2 inches wide: 6.5^3 × 2.2 = 604.18 in^4.
Add together the previous two steps' answers: 1,024 + 604.18 = 1,628.18 in^4.
Divide this sum by 12: 1,628.18 / 12 = 135.68, or just over 135 in^4. This is the H-beam's area moment of inertia.