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Steel weight drives a fabrication estimate. It sets the material cost, the lifting plan, the transport and often the price of galvanizing. Most of it can be worked out from the drawing with a few rules, as long as you remember what those rules leave out.
Start from 7.85
Carbon steel weighs 7,850 kg per cubic metre. In workshop units that means:
- A steel bar or section weighs 7.85 kg per metre for every 1,000 mm² of cross-section area.
- Steel plate weighs 7.85 kg per square metre for every millimetre of thickness. A 2 × 1 m sheet of 10 mm plate weighs 157 kg.
Stainless steel is 1–2% heavier, at 7,930 to 8,000 kg/m³, and aluminium weighs about a third as much as steel, at 2,700 kg/m³.
Shortcuts for common shapes
Scroll the table sideways to see all columns on a small screen.
| Shape | Carbon steel, kg/m | Example |
|---|---|---|
| Round bar, diameter D | 0.006165 × D² | Ø50: 15.4 kg/m |
| Square bar, side A | 0.00785 × A² | 40 × 40: 12.6 kg/m |
| Flat bar, W × T | 0.00785 × W × T | 100 × 10: 7.85 kg/m |
| Round tube, D × t | 0.02466 × (D − t) × t | 60.3 × 3.9: 5.43 kg/m |
All sizes are in millimetres.
Rolled sections need the table
Angles, channels, beams and hollow sections have rounded corners and root radii, so working them out from their outline sizes is only approximate. A cold-formed 100 × 50 × 4 box section weighs about 8.6 kg/m, while its sharp-cornered outline gives 8.9 kg/m. For an order or a lifting plan, take the mass per metre from the section tables of the standard, such as EN 10219 for cold-formed hollow sections or EN 10056 for angles.
From drawing weight to order weight
The weight of the finished part is not the weight you buy. Allow for:
- Offcuts. Bars and sections come in stock lengths, often 6 or 12 m, and the last piece of each length is often scrap. Plan the cut list against stock lengths before you order.
- Plate nesting. Parts cut from plate leave a skeleton. Rectangles nest well, but parts with curves can waste a fifth of the plate or more.
- Weld metal. Usually a small share of the total, often 1–2% on fabricated frames, and more on heavy plate with deep welds.
- Coatings. Paint adds little weight. Hot-dip galvanizing adds a few percent, and more on thin material, which matters because galvanizers charge by weight.
Painting area
Blasting and painting are priced by area, not weight. The outside area of a member is its outside perimeter times its length: a 100 × 50 box section has 0.3 m² per metre. Light sections have far more surface per tonne than heavy ones, so a frame of light members costs more to paint per tonne than a heavy plate structure.
Lifting
Use the calculated weight as the starting point for a lift, then add the slings, shackles and any spreader beam. Round up, mark the weight on large assemblies, and weigh the load or check the drawing weight again when it comes close to the crane's capacity.
