Round tube ⌀ 125 × 4 in steel — 71.62 kg a piece

Only the section formula changes with shape

Plate and flat bar are thickness × width, square bar is side², round bar is π/4 × diameter². Tubes are the outside section minus the inside one: π/4 × (outside² − inside²) for round tube, outside² − inside² for box section, where the inside size is the outside less the wall on both faces, outside − 2t. Subtract the wall only once and you get a tube with half the wall and roughly half the weight — that factor of two is the single most common slip in this calculation.

• π/4 × (D² − (D − 2t)²)1520.53 mm²
• 1520.53 mm² ÷ 10⁶ × 7850 kg/m³11.936 kg/m

Box sections have rounded corners and come out lighter

This table treats box sections as having square corners. Real ones are rolled with a radius, which removes metal at the four corners, so the figures here run 2 to 3 % above the published mass: a 50 × 50 × 2.3 is listed at 3.34 kg/m while the square-cornered calculation gives 3.44 kg/m. The corner radius varies by mill and is not part of the size designation, so it cannot go into the formula — it is left out, and that omission is stated here rather than hidden. Round tube has no corners, so the same calculation matches the published value exactly.

Nearby cells

Same shape, other sizes

Worth knowing

• Weight per metre (kg/m) is section in mm² ÷ 10⁶ × 7850 — only the section formula changes with shape.
• For plate, thickness × 7.85 is the weight per m²: 47.1 kg/m² at 6 mm, twice that at 12 mm.
• For tube, subtract the inside section from the outside one; the inside size is the outside less 2t.
• Doubling a dimension quadruples the section, while length is directly proportional to weight.

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