Rebar weight is calculated using the formula: Weight (kg) = (D² ÷ 162.27) × Length (m), where D is the bar diameter in millimetres. For example, a 12mm bar that is 6 metres long weighs (144 ÷ 162.27) × 6 = 0.888 × 6 = 5.33 kg. The constant 162.27 is derived from the density of steel (7850 kg/m³) and the circular area formula.

A 12mm diameter rebar weighs approximately 0.888 kg per metre. Using the formula D²/162.27: 12² = 144, divided by 162.27 = 0.888 kg/m. For a standard 6-metre bar, the total weight is 5.33 kg per bar. For 10 bars of 6m length, the total weight would be 53.3 kg.

Standard metric rebar sizes in the UK (to BS 4449) are: 6mm, 8mm, 10mm, 12mm, 16mm, 20mm, 25mm, 32mm, and 40mm. The most commonly used in general construction are T10, T12, T16, and T20. High-yield deformed bars (grade B500B or B500C) are standard for structural reinforcement. Bars are typically stocked in 6-metre lengths.

The constant 162.27 comes from combining the steel density and circular area formula: π/4 × ρ × (1/1000)², where ρ = 7850 kg/m³ (density of steel). Simplified: 1 ÷ (π/4 × 7850 ÷ 1,000,000) ≈ 162.27. Some references round this to 162, giving the approximation D²/162. This is accurate to within 0.17% and is widely used in construction practice.

To convert kilograms to metric tonnes, divide by 1000. For example, 2500 kg of rebar = 2.5 tonnes. Steel is often ordered and priced by the tonne for large projects. For smaller quantities, pricing is usually per length (per bar) or per kg. Always confirm with your supplier whether they quote in metric tonnes (1000 kg) or imperial tons (1016 kg).

Standard stock length for rebar in the UK is 6 metres. Some suppliers also stock 12-metre lengths for specific applications. Rebar can be cut to custom lengths (usually with a minimum order) or bent to shape at the fabricator. For project scheduling, always calculate based on the actual cut length needed, accounting for laps, hooks, and bends which add to the total bar length required.

To estimate rebar quantity: (1) Calculate the total linear metres of bar needed from your drawings, counting each bar and its length. (2) Multiply by the weight per metre for your bar size. (3) Add 5–10% waste allowance for cuts and laps. For slabs, a common rule of thumb is 80–100 kg of rebar per cubic metre of concrete for typical reinforced slabs, though this varies considerably by design. Always follow the structural engineer’s specification.

The most common rebar grades in the UK are B500B and B500C (to BS 4449:2005), both with a characteristic yield strength of 500 N/mm². B500C has higher ductility (Class C) than B500B (Class B), making it preferable in seismic zones. Older Grade 460 (to BS 4449:1997) may be found in existing structures. In the US, ASTM A615 Grade 60 (60,000 psi / 414 MPa) is standard. Weight per metre is the same regardless of grade, as it depends only on density and diameter.