Steel weight calculation: the D²/162 formula and a bar weight table
The weight of a round steel bar per metre is D² ÷ 162 kilograms, where D is the bar diameter in millimetres. A 12 mm bar therefore weighs 144 ÷ 162 = 0.889 kg per metre. Every bar bending schedule and steel estimate in India is built on this one formula.
Where D²/162 comes from
Weight = volume × density. One metre of bar of diameter D mm has a volume of π/4 × (D/1000)² × 1 m³, and steel weighs 7,850 kg/m³:
Weight per metre = π/4 × D² × 7850 ÷ 1,000,000 = D² × 0.006165 kg/m = D² ÷ 162.2
Rounding 162.2 to 162 is the familiar shortcut; the difference is about 0.1 %.
Weight of steel bars — kg per metre
| Diameter | D²/162 (kg/m) | Exact (kg/m) | One 12 m bar (kg) |
|---|---|---|---|
| 6 mm | 0.222 | 0.222 | 2.66 |
| 8 mm | 0.395 | 0.395 | 4.74 |
| 10 mm | 0.617 | 0.617 | 7.40 |
| 12 mm | 0.889 | 0.888 | 10.65 |
| 16 mm | 1.580 | 1.578 | 18.94 |
| 20 mm | 2.469 | 2.466 | 29.59 |
| 25 mm | 3.858 | 3.853 | 46.24 |
| 28 mm | 4.840 | 4.834 | 58.00 |
| 32 mm | 6.321 | 6.313 | 75.76 |
| 36 mm | 8.000 | 7.990 | 95.88 |
| 40 mm | 9.877 | 9.865 | 118.38 |
Worked example
A slab needs 420 m of 10 mm bars and 180 m of 8 mm bars.
- 10 mm: 100 ÷ 162 = 0.617 kg/m × 420 m = 259.3 kg
- 8 mm: 64 ÷ 162 = 0.395 kg/m × 180 m = 71.1 kg
- Total = 330.4 kg, or 0.330 tonne, before wastage and laps that are not already in the lengths.
Things that change the answer
- Lengths must be cutting lengths — with hooks, bends and laps — not the clear span. That is what a bar bending schedule works out.
- Rolling tolerance. IS 1786 allows the actual mass per metre of TMT bars to vary from the nominal within stated limits; bills are normally made on nominal mass.
- Wastage from cutting is usually allowed as a small percentage in the estimate, not in the measured bill.
CivilOS's bar bending schedule computes every cutting length to IS 2502 and SP 34, multiplies it by D²/162 and totals the steel by diameter, each row showing its own formula.
Make a bar bending schedule in CivilOS →Frequently asked questions
What is the formula for steel weight?
Weight in kg per metre = D² ÷ 162, where D is the bar diameter in mm. It comes from the volume of one metre of bar multiplied by the density of steel, 7,850 kg/m³.
What is the weight of a 12 mm bar per metre?
0.889 kg per metre (144 ÷ 162). A full 12 m length weighs about 10.65 kg.
What is the weight of a 16 mm bar per metre?
1.58 kg per metre (256 ÷ 162). A full 12 m length weighs about 18.9 kg.
Why 162?
Because π/4 × 7,850 ÷ 1,000,000 = 0.006165, and 1 ÷ 0.006165 ≈ 162.2. It is the density of steel and the area of a circle folded into one number.