Steel Quantity Calculator
Rebar weight by bar size, length, and count — using the standard d²/162 formula
Frequently Asked Questions
Rebar (reinforcement steel) weight is calculated using a single standard formula that applies universally regardless of bar size — diameter squared, divided by 162, gives weight per metre in kilograms — but knowing the formula doesn't remove the need to correctly total up every bar size and length actually used in a structure.
The d²/162 formula, and why bar size matters so much
The formula — weight per metre (kg) = diameter² (mm) ÷ 162 — is a standard, universally-used approximation for steel rebar density and cross-sectional area, and it applies the same way across every common bar size (6mm, 8mm, 10mm, 12mm, 16mm, 20mm, 25mm). Because the formula squares the diameter, weight per metre grows much faster than diameter itself: a 20mm bar isn't just "bigger" than a 10mm bar, it's roughly four times heavier per metre, since doubling the diameter quadruples the diameter-squared term. This is why substituting a different bar size than specified — even one that looks similar — can change total steel weight (and therefore cost, and structural capacity) by far more than the size difference alone suggests.
A worked example
The same length and count of bars, calculated at two different diameters, produces weights that don't scale linearly with the diameter change — going from a smaller to a larger bar size roughly quadruples weight per metre for a doubling of diameter, which is a much bigger jump than most people intuitively expect from what sounds like a modest size increase.
How this connects to your other construction planning
Steel quantity calculated here feeds directly into the Roof Slab Calculator, which uses a simplified kg-per-square-foot ratio rather than counting individual bars — use this calculator when you have an actual bar schedule (specific diameters, lengths, and counts from a structural drawing) and want an exact weight, and the roof slab calculator's ratio-based estimate when you don't yet have that level of design detail.
Common mistakes
The most common mistake is applying the d²/162 formula correctly but then working from an incomplete or outdated bar count — always total steel weight from the final, engineer-approved bar schedule rather than an early estimate, since bar sizes and counts often change between initial design and final structural drawings.