Rebar weight formula: d² ÷ 162
Weight (kg/m) = d² ÷ 162, with the bar diameter d in mm
Site engineers and bar benders use this rule to estimate reinforcement quantities. A 12 mm bar weighs 144 ÷ 162 = 0.889 kg per metre, and a 12 m length about 10.7 kg.
Where 162 comes from
- Area of a round bar: π d² ÷ 4 mm².
- Volume of one metre of bar: π d² ÷ 4 × 10⁻⁶ m³.
- Mass at 7850 kg/m³: 7850 × π ÷ 4 × 10⁻⁶ × d² = 0.0061654 × d² kg/m.
- 1 ÷ 0.0061654 = 162.2, so the mass is d² ÷ 162.2, rounded to d² ÷ 162.
How accurate is it?
| Bar | d² ÷ 162 | Exact (7850 kg/m³) | Nominal mass |
|---|---|---|---|
| 6 mm | 0.222 | 0.222 | 0.222 |
| 8 mm | 0.395 | 0.395 | 0.395 |
| 10 mm | 0.617 | 0.617 | 0.617 |
| 12 mm | 0.889 | 0.888 | 0.888 |
| 14 mm | 1.210 | 1.208 | 1.210 |
| 16 mm | 1.580 | 1.578 | 1.580 |
| 20 mm | 2.469 | 2.466 | 2.470 |
| 25 mm | 3.858 | 3.853 | 3.850 |
| 28 mm | 4.840 | 4.834 | 4.830 |
| 32 mm | 6.321 | 6.313 | 6.310 |
| 40 mm | 9.877 | 9.865 | 9.860 |
| 50 mm | 15.432 | 15.413 | 15.400 |
The rule is about 0.1 % high, well inside the rolling tolerance of reinforcing bars. Ribbed (deformed and TMT) bars are specified by a nominal diameter that gives the nominal mass, so the same rule applies to them.
US bar sizes
US bar numbers are the diameter in eighths of an inch: a #4 bar is 4⁄8 = ½ in. The equivalent rule in US units is weight (lb/ft) = 2.67 × d² with d in inches, so #4 weighs 2.67 × 0.25 = 0.668 lb/ft. See the rebar weight chart for every size.
Quantity take-off
Total weight = Σ (number of bars × cut length × d² ÷ 162). Add an allowance for laps and wastage as your project specification requires. For a single size, the calculator or the page for each bar, e.g. 12 mm rebar, gives the weight for any length and count.