Steel weight formula
Weight = cross-sectional area × length × density
Every steel weight calculation comes down to this one line. With the density of carbon steel, 7850 kg/m³, it becomes:
weight (kg) = area (mm²) × length (m) × 0.00785
weight (lb) = area (in²) × length (ft) × 3.40
The factor 0.00785 is 7850 kg/m³ × 10⁻⁶ m² per mm². In US units, 490 lb/ft³ ÷ 144 in²/ft² = 3.40 lb per foot for each square inch of cross-section.
Cross-sectional area of each shape
| Shape | Area |
|---|---|
| Plate / sheet | A = t × w |
| Round bar | A = π d² / 4 |
| Square bar | A = a² |
| Flat bar | A = w × t |
| Hexagon bar | A = (√3 / 2) s² |
| Pipe / round tube | A = π (D − t) t |
| Square tube | A = a² − (a − 2t)² |
| Rectangular tube | A = a b − (a − 2t)(b − 2t) |
| Angle | A = t (a + b − t) |
| Channel (sharp corners) | A = 2 b tf + (h − 2 tf) tw |
| I-beam (sharp corners) | A = 2 b tf + (h − 2 tf) tw |
Rolled beams, channels and angles have rounded fillets and, for some standards, sloping flanges, so the simple formulas above slightly under- or over-estimate them. Use the published weight per metre for rolled sections; the beam, channel and angle charts list it for every size.
Worked examples
| Item | Calculation | Result |
|---|---|---|
| Plate 10 × 1250 × 2500 mm | 10 × 1.25 × 2.5 × 7.85 | 245.3 kg |
| Flat bar 50 × 6 mm | 50 × 6 × 0.00785 | 2.355 kg/m |
| Angle 50 × 50 × 5 mm (sharp corners) | 5 × (50 + 50 − 5) × 0.00785 | 3.729 kg/m |
| Pipe 60.3 × 3.9 mm (2" Sch 40) | π × (60.3 − 3.9) × 3.9 × 0.00785 | 5.42 kg/m |
The angle example gives 3.73 kg/m; the rolled ISA 50 x 50 x 5 is listed at 3.79 kg/m in IS 808:2021 because its root fillet adds material. That is why rolled sections should be looked up rather than calculated.
Shortcuts worth remembering
- Plate: 7.85 kg per m² per mm of thickness; 40.8 lb per ft² per inch.
- Round bar: d² / 162 kg/m with d in mm (why 162).
- Pipe: 0.02466 × (OD − t) × t kg/m with mm; 10.69 × (OD − t) × t lb/ft with inches.
- Square bar: a² × 0.00785 kg/m.
Other materials
Replace 7850 with the density of the metal: stainless steel about 7930 to 8000 kg/m³, aluminum 2700, copper 8960, brass 8500. See density of steel and other metals, or let the calculator do it.
Frequently asked questions
What is the formula for the weight of steel?
Weight = volume × density. For steel, weight (kg) = cross-sectional area (mm²) × length (m) × 0.00785, because steel weighs 7850 kg/m³ and 1 mm² × 1 m = 10⁻⁶ m³.
What is the formula for steel plate weight?
Weight (kg) = thickness (mm) × width (m) × length (m) × 7.85. In US units, weight (lb) = thickness (in) × width (ft) × length (ft) × 40.8.
What is the formula for steel pipe weight?
Weight (kg/m) = 0.02466 × (OD − t) × t with OD and wall thickness t in mm. In US units, lb/ft = 10.69 × (OD − t) × t with OD and t in inches.