When preparing a quote, planning transport or keeping stock, the first question is usually the same: how much does this material weigh? Steel is mostly bought and sold by the kilogram or tonne, so getting quickly and accurately from size to weight is fundamental. The good news is that every calculation rests on one simple rule.
The basic rule: volume × density
The weight of a bar is its volume multiplied by the density of the material. For bars, volume means cross-sectional area × length. In practice the density of steel is taken as 7.85 g/cm³ (7,850 kg/m³).
If we take sizes in millimetres and length in metres, the calculation becomes very simple: a bar with a 1 mm² cross-section and a length of 1 m has a volume of 1,000 mm³, or 1 cm³. That weighs 7.85 grams, or 0.00785 kg.
Round bar
The area of a circle is π × d² ÷ 4. Multiplied by 0.00785, the factor becomes 0.006165:
Example: for a Ø50 mm bar, 50² × 0.006165 = 15.41 kg/m. A 6-metre length weighs 15.41 × 6 = about 92.5 kg.
Square and flat bar
For a rectangular section the area is simply the product of the two sides; a square is the same with equal sides.
Example: a 40 × 40 mm square: 1,600 × 0.00785 = 12.56 kg/m. A 100 × 20 mm flat: 2,000 × 0.00785 = 15.70 kg/m.
Hexagon bar
For hexagon bar the size is the distance between two opposite flat sides, also called across flats (SW). The area of a regular hexagon is (√3 ÷ 2) × s², or about 0.866 × s².
Example: SW 36 hexagon: 36² × 0.0068 = 8.81 kg/m. Entering the across-corners size is a common mistake and inflates the result by about 33%.
Tube
The cross-section of a tube is a ring: the outer circle minus the inner circle. Since the inside diameter is D − 2t, the area simplifies to π × (D − t) × t.
Example: for an 88.9 × 5 mm tube, (88.9 − 5) × 5 × 0.02466 = 10.35 kg/m.
Plate and sheet
For plate it is usual to work from the weight per square metre: 1 m² of steel plate 1 mm thick weighs 7.85 kg.
Example: a 1,500 × 3,000 mm plate, 10 mm thick, weighs 1.5 × 3 × 10 × 7.85 = 353.25 kg.
Metals other than steel
The factors above are for steel. To find the weight of the same size in another material, simply multiply the steel weight by that material's density factor. Copper, for example, has a factor of 1.131, so a Ø50 mm copper bar weighs about 15.41 × 1.131 = 17.43 kg/m.
Why does theoretical weight differ from weighed weight?
The value from the formulas is the theoretical weight. In rolled and forged products, sizes vary within the tolerances the standards allow; corner radii, ovality and wall-thickness variation can also shift the real weight by a few per cent. So:
- You can safely use theoretical weight for planning, transport and preliminary quotes.
- Commercially, either theoretical or weighed weight may apply; it is good practice to agree which one at the quoting stage.
- Watch your units: a diameter entered in centimetres makes the result 100 times too small.