The rebar weight formula
Steel has a density of about 7,850 kg/m³. For a round bar of diameter D in millimetres, the weight per metre simplifies to:
Total weight (kg) = D² ÷ 162 × length (m) × number of bars
The number 162 comes from 1,000,000 ÷ (7,850 × π ÷ 4) ≈ 162.2. It is accurate to within 0.2% of the nominal mass in BS 4449 and ASTM A615 tables.
Worked example
40 bars of 16 mm at 12 m: 16² ÷ 162 = 1.580 kg/m. 1.580 × 12 × 40 = 758.5 kg. Add 5% for laps and offcuts and you order about 0.80 t.
Unit weight table
| Diameter | kg per metre | kg per 12 m bar | Bars per tonne (12 m) |
|---|
Frequently asked questions
What is the weight of a 12 mm bar per metre?
12² ÷ 162 = 0.889 kg/m, so a standard 12 m bar weighs about 10.67 kg.
How much lap allowance should I add?
Lap lengths are typically 40–50 times the bar diameter. As a quick estimate, 5–8% covers laps and cutting waste for slabs; detailed bar bending schedules give exact figures.
Is the formula valid for deformed (ribbed) bars?
Yes. Ribbed bars are sold by nominal diameter, and the nominal mass per metre matches D²/162 closely.
How many kg of steel per m³ of concrete?
Rough ranges: slabs 80–120 kg/m³, beams 150–250 kg/m³, columns 180–300 kg/m³, footings 60–100 kg/m³. Use these for early budgets only.