About this calculator
Steel reinforcement is bought and estimated by weight, but drawn and cut by length. This calculator converts between the two using the nominal mass of the bar. It is the familiar site rule weight per metre = D² ÷ 162, worked out exactly.
Formula
m = (πd² ÷ 4) × 7850 kg/m ≈ d² ÷ 162.2 kg/m
d = nominal diameter (in m for the first form, in mm for the second). 7850 kg/m³ is the density of steel used by reinforcement standards to define nominal mass.
Total weight = weight per metre × bar length × number of bars × (1 + wastage).
Worked example
Ten 12 mm bars, each 12 m long:
- Weight per metre = 12² ÷ 162.2 = 0.888 kg/m
- Weight per bar = 0.888 × 12 = 10.65 kg
- Total = 10.65 × 10 = 106.5 kg
Nominal masses from this formula: 8 mm 0.395 · 10 mm 0.617 · 12 mm 0.888 · 16 mm 1.58 · 20 mm 2.47 · 25 mm 3.85 · 32 mm 6.31 kg/m — the same values listed in common rebar standards such as BS 4449 and ISO 6935-2.
Assumptions and limitations
- Uses nominal mass. Standards allow a tolerance on actual mass per metre (typically a few percent), so delivered weight can differ.
- Wastage, laps and hooks must be included by you, either in the lengths or in the wastage allowance.
- This estimates quantities only. Reinforcement design (sizes, spacing, cover, lap lengths) must follow your concrete design code and a structural engineer’s drawings.
References
- BS 4449 and ISO 6935-2: nominal mass of reinforcing bars based on 7.85 kg/dm³.