149 steel cells — a 6 mm plate weighs 47.1 kg per m²

Weight is volume times density

Steel sections have a constant cross-section, so the volume of one metre is fixed by that section and its weight is that volume times the density of steel, 7850 kg/m³. The weight per metre is therefore section in mm² ÷ 10⁶ × 7850, and the three shortcuts used on site are just that formula tidied up: thickness × 7.85 for plate, 0.006165 × diameter² for round, 0.00785 × side² for square. Knowing where those numbers came from beats memorising them — any size can be worked out without a table.

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.

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.

H-beams and angles are left out

H-beams, I-beams, angles, channels, lipped C sections and tees have a mass per metre that only a standards table can give. What decides that mass is the fillet where web meets flange and the taper inside the flange, and neither appears in a designation like H-200 × 100 × 5.5 × 8 — the visible dimensions do not determine the area. Those rows could be copied in, but then a single mistyped figure would go unnoticed forever: the property that every number here can be re-derived from the density and π would break at that row. So only shapes that come out of pure geometry are listed.

Platet × w

Flat bart × w

Square bara²

Round barπ/4 × d²

Round tubeπ/4 × (D² − (D − 2t)²)

Square tubea² − (a − 2t)²

Rectangular tubea × b − (a − 2t)(b − 2t)

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.