Structural steel cut list basics: steel weight per metre formulas
Structural steel cut list helpers: work out steel weight in kg/m from section dimensions using 7,850 kg/m³ for flat bar, round bar, plate, SHS/RHS and CHS, with worked examples.
Carbon steel catalogues in Australia, such as InfraBuild's Know Your Steel, work out mass using a density of 7,850 kg/m³. That means 1 mm² of cross-section weighs 0.00785 kg per metre. Work out the cross-sectional area in mm², multiply by 0.00785, and you have kg/m.
Formulas (dimensions in mm)
| Section | kg/m |
|---|---|
| Flat bar W × T | W × T × 0.00785 |
| Round bar, diameter D | π × D² ÷ 4 × 0.00785 |
| Plate, thickness T | T × 7.85 kg per m² |
| SHS/RHS B × D × T (square corners, approx) | (2 × (B + D) − 4 × T) × T × 0.00785 |
| CHS, outside diameter OD × wall T | π × (OD − T) × T × 0.00785 |
Worked examples
- 50 × 6 flat bar: 50 × 6 × 0.00785 = 2.355 kg/m
- 20 mm round bar: π × 400 ÷ 4 × 0.00785 ≈ 2.466 kg/m
- 10 mm plate: 10 × 7.85 = 78.5 kg/m², so a 1.2 × 2.4 m sheet is about 226 kg
- 50 × 50 × 3 SHS: (200 − 12) × 3 × 0.00785 ≈ 4.43 kg/m (square-corner estimate)
Use the supplier's tables for real sections
Hollow sections have rounded corners, so the square-corner formula reads a little high. Rolled sections (UB, UC, PFC, angles) have published kg/m values. For quoting and ordering, use the supplier's catalogue figure and keep the formula for flat bar, plate and odd sizes. Catalogues also note rolling tolerances, so weighbridge weights can differ a little from calculated ones.
From kg to a price
Total kg = kg/m × length (m) × quantity. If your supplier prices by the kilo, multiply by the $/kg; if they price by the length, use that instead. Add an allowance for offcuts.
Sources
Related guides
- What goes in a fabrication ITP (AS/NZS 5131 projects)
- How to quote a job with GST (Australia)
- Welding tickets in Australia: what builders actually ask for
General information only, summarised from the official sources above as at the review date. It isn't legal, tax, WHS or engineering advice. Rules change and your situation may differ, so check the source or ask a professional.