Steel bar is priced by weight and specified by dimension, so converting between the two is a daily calculation in every purchase department. The formulas are simple, but the constants are worth understanding rather than copying.
Where the constants come from
Weight per metre is cross-sectional area multiplied by density. Steel density is taken as 7,850 kg/m³. Working in millimetres and metres, every formula reduces to dimension squared times a constant.
- Round bar: area = π/4 × d². Multiplied out, weight per metre = d² × 0.006165 kg, where d is diameter in mm.
- Hex bar: area = (√3/2) × AF². Weight per metre = AF² × 0.006798 kg, where AF is the across-flats dimension in mm.
- Square bar: area = a². Weight per metre = a² × 0.00785 kg, where a is the side in mm.
A worked example
A 20 mm round bar, 3 metres long: 20² × 0.006165 = 2.466 kg per metre, so the bar weighs 7.398 kg. For 500 such bars, that is 3,699 kg — 3.699 MT.
A 20 mm hex bar of the same length: 20² × 0.006798 = 2.719 kg per metre, or 8.158 kg per bar. Hex is roughly 10% heavier than round at the same nominal size, because the hexagon encloses more area than the inscribed circle.
Four mistakes that cause wrong quantities
- Using the across-corners dimension for hex instead of across-flats. Hex bar is always specified across flats; across corners is about 15.5% larger and will overstate weight substantially.
- Using the round constant for hex bar. It understates weight by around 10%.
- Mixing units — length in mm in one place and metres in another. Keep dimensions in mm and length in metres throughout.
- Expecting the calculated figure to match the weighbridge exactly. It will not, and should not.
Why calculated weight never matches actual exactly
These formulas use the nominal dimension. A cold drawn bar to an h-tolerance is never larger than nominal and is usually slightly under it, so actual weight is typically a little below calculated. Density also varies marginally between grades, and cut length has its own tolerance.
Use the calculation for estimating, ordering and planning. Use the weighbridge for invoicing. On our dispatches, incoming and outgoing weights are both taken on the public weighbridge and the slip is filed against the challan.
Written by the technical team at Kalika Enterprises, Waluj MIDC. Guidance here is general; the values that govern your part are those on the test certificate supplied with your consignment.