Rebar weight calculation: why d²/162 works — and the kg/ft table

Reinforcement is bought by weight but measured by length. The bridge between the two is the unit weight of each bar diameter.

The derivation

Steel weighs about 7,850 kg/m³. A bar of diameter d (mm) has a cross-section of π d² / 4 mm², which is π d² / 4 × 10⁻⁶ m².

Weight of 1 m = π d² / 4 × 10⁻⁶ × 7,850 = 0.006165 d² kg

and 1 / 0.006165 = 162.2, so

unit weight = d² / 162 kg per metre.

For feet, divide by 3.2808: d² / 533 kg per foot.

The table

Worked example

A slab panel needs 48 bars of 10 mm, each 4.2 m long, plus 52 bars of 12 mm at 3.6 m:

• 10 mm: 48 × 4.2 = 201.6 m × 0.617 = 124.4 kg
• 12 mm: 52 × 3.6 = 187.2 m × 0.888 = 166.2 kg

Add a lapping and wastage allowance (commonly 5–10 % depending on detailing) — or work bar by bar with a bar bending schedule.

Common mistakes

• Using nominal length instead of cutting length — hooks and laps add length; bends reduce it slightly.
• Ignoring stock length — bars come in 12 m lengths; offcuts become wastage unless re-used.
• Mixing units — a length in feet multiplied by a kg/m figure overstates weight by 3.28 times.
• Rounding unit weights too early — keep three decimals until the final total.

Rolling tolerance

Actual bar weights vary within the tolerance allowed by the standard (BDS ISO 6935-2 / ASTM A615). For procurement, weigh a sample of each delivery.

In EstiMatrix

The rebar take-off applies these unit weights automatically and shows the multiplier next to each diameter.