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Steel Weight Formula

All steel weight formulas in one place, with the reason they work and a worked example for each. Every formula uses millimetres for sizes and gives kilograms.

The One Rule Behind Every Formula

Weight = Volume × Density

Steel has a density of 7,850 kg/m³, which is 7.85 g/cm³. A section of 1 mm² that is 1 m long has a volume of 1 cm³, so it weighs 7.85 g, which is 0.00785 kg. That is why so many steel formulas end with × 0.00785: you work out the cross-section area in mm² and multiply.

Weight (kg/m) = Cross-section area (mm²) × 0.00785

Steel Weight Formulas by Shape

Shape	Formula
Plate / sheet	L (m) × W (m) × t (mm) × 7.85 = kg
Round bar / rod / TMT	D² ÷ 162 = kg/m
Round pipe	(OD − t) × t × 0.02466 = kg/m
Square / rectangular pipe	[A × B − (A − 2t)(B − 2t)] × 0.00785 = kg/m
Angle	(A + B − t) × t × 0.00785 = kg/m
Channel	[H × tw + 2(B − tw) × tf] × 0.00785 = kg/m
I-beam	[2 × B × tf + (H − 2tf) × tw] × 0.00785 = kg/m
Flat bar	W × t × 0.00785 = kg/m
Square bar	A² × 0.00785 = kg/m

L = length, W = width, t = thickness, D = diameter, OD = outside diameter, A/B = sides or legs, H = depth, tw = web thickness, tf = flange thickness.

Why D² ÷ 162 for Rods?

The area of a circle is π × D² ÷ 4. Multiply by 0.00785: π ÷ 4 × 0.00785 = 0.006165, and 1 ÷ 0.006165 = 162.2. So weight per meter = D² ÷ 162. A 12 mm bar: 144 ÷ 162 = 0.889 kg/m.

Why 0.02466 for Pipes?

A pipe is a ring: π × (OD − t) × t gives its area. π × 0.00785 = 0.02466, so weight per meter = (OD − t) × t × 0.02466. A 48.3 × 3.2 mm pipe: 45.1 × 3.2 × 0.02466 = 3.56 kg/m.

Worked Examples

Item	Calculation	Weight
MS plate 2500 × 1250 × 10	2.5 × 1.25 × 10 × 7.85	245.31 kg
12 mm TMT bar, 12 m	(144 ÷ 162) × 12	10.67 kg
Angle 50 × 50 × 6, 6 m	(100 − 6) × 6 × 0.00785 × 6	26.56 kg
Flat 50 × 6, 6 m	50 × 6 × 0.00785 × 6	14.13 kg
Square pipe 40 × 40 × 2, 6 m	304 × 0.00785 × 6	14.32 kg

Other Metals

Swap 7.85 for the metal's density, or multiply the steel result by (density ÷ 7.85): stainless steel 304 = 7.93, aluminium = 2.70, copper = 8.96, brass = 8.50.

Frequently Asked Questions

What is the formula for steel weight?

Weight = volume × density. For steel sections in mm, weight per meter = cross-section area (mm²) × 0.00785.

Why is 7.85 used in steel weight calculation?

Because the density of steel is 7.85 g/cm³ (7,850 kg/m³). For a plate, length (m) × width (m) × thickness (mm) × 7.85 gives kg directly.

What is the formula for steel rod weight?

D² ÷ 162 gives kg per meter, where D is the diameter in mm.

How do I calculate steel weight per foot?

Calculate kg per meter and multiply by 0.3048.