Square tube 40 × 40 × 5 in steel — 32.97 kg a piece
Square tube 40 × 40 × 5 weighs 32.97 kg. Its 700 mm² section times a density of 7850 kg/m³ gives 5.495 kg/m, so 100 pieces come to 3297 kg and a tonne holds 30.
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.
- a² − (a − 2t)²700 mm²
- 700 mm² ÷ 10⁶ × 7850 kg/m³5.495 kg/m
- Size
- 40 × 40 × 5 mm
- Section formula
- a² − (a − 2t)²
- Cross-section
- 700 mm²
- Weight per metre
- 5.495 kg/m
- Inside size
- 30 mm
- If it were solid
- 12.56 kg/m
- Saved by being hollow
- 56.3 %
- Length of one
- 6 m
- Weight of one
- 32.97 kg
- 100 pieces
- 3297 kg
- Pieces per tonne
- 30
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.
Common questions
Q. How much does one Square tube 40 × 40 × 5 weigh?
32.97 kg — 5.495 kg/m times its length. Ten pieces come to 329.7 kg and a hundred to 3297 kg.
Q. How many pieces make a tonne?
30. That is 1000 divided by 32.97 kg, and 100 pieces weigh 3.297 t.
Q. Where does 5.495 kg/m come from?
The section is a² − (a − 2t)², which gives 700 mm²; convert that to m² and multiply by a density of 7850 kg/m³. With the dimensions in hand you get the same number without any table.