How metal weight is calculated
Weight (kg) = cross-section area (mm²) × length (mm) × density (g/cm³) ÷ 1,000,000
The only thing that changes between shapes is the cross-section area:
- Sheet / plate / flat: width × thickness
- Round bar: π × d² ÷ 4
- Pipe / tube: π × (OD² − ID²) ÷ 4, where ID = OD − 2 × wall thickness
- Square bar: side²
- Rectangular tube: (A × B) − (A − 2t) × (B − 2t)
- Angle: t × (A + B − t)
- Hex bar: 0.866 × (across flats)²
The quick rule for steel bars
On site, steel round bar (TMT, rod) weight is often worked out as d² ÷ 162 kg per metre, where d is the diameter in mm. A 12 mm bar: 144 ÷ 162 = 0.888 kg/m. This comes from the same formula with density 7.85.
Worked example
An MS pipe with 48.3 mm outside diameter and 3.2 mm wall, 6 m long: ID = 41.9 mm; area = π × (48.3² − 41.9²) ÷ 4 = 453.9 mm²; weight = 453.9 × 6000 × 7.85 ÷ 1,000,000 = 21.4 kg (3.56 kg/m).
Why your invoice weight may differ
Mills roll to tolerances, so actual sections can be a few percent heavier or lighter than the nominal size. Galvanising adds weight. Traders may sell “by theoretical weight” or “by actual weight” — ask which before you compare quotes.
Questions people ask
What density should I use for stainless steel?
How do I calculate weight per metre?
Does the calculator work for galvanised (GI) pipe?
Sources
- IS 1161 (structural steel tubes) and IS 1239 (steel pipes) — Bureau of Indian Standards
- Densities: standard engineering reference values
Results are estimates to help you plan. Always confirm important numbers with the seller, carrier or authority concerned. Last reviewed: November 2026.