The cost of a concrete structure depends heavily on how much concrete and reinforcing steel it contains. Steel is far more expensive than concrete, so knowing its quantity accurately up front goes a long way toward knowing the cost of the whole project.
Here are the methods commonly used to estimate rebar, from quick rules of thumb to a detailed takeoff. The examples use metric units.
Method 1: Rule of thumb by concrete volume
For a rough estimate, a common rule of thumb is about 80 kg of rebar per cubic meter of slab. For a slab measuring 10 m × 10 m × 0.2 m:
- Concrete = 10 × 10 × 0.2 = 20 m³
- Rebar = 80 kg × 20 = 1,600 kg (1.6 metric tons)
This method is often used to get a rough idea of the weight and cost of the steel in a building.
Method 2: Percentage of concrete volume
Another rule of thumb takes steel as 1% of the slab volume. For the same 20 m³ slab:
- Steel = 1% of 20 m³ = 0.2 m³
- Weight = 0.2 m³ × 7,850 kg/m³ = 1,570 kg (1.57 metric tons)
Both rules of thumb depend on experience and vary from one building to another. You need enough data before you can set reliable ratios, so it's best to base them on a previously built project of a similar type — a practice known as benchmarking.
Method 3: Measure from the drawings or a bar bending schedule
This is the detailed method. First, measure the total length of each bar diameter, including straight bars, laps, and bends. Then multiply each total length by the unit weight of that bar size to get the total weight of steel.
For example, if the total length of 20 mm bars is 100 m, the weight is 100 m × 2.47 kg/m = 247 kg.
Method 4: Use estimating software
Takeoff software such as PlanSwift and Bluebeam can calculate bar weights directly once you enter the area and bar spacing. The results are close to exact; add a waste allowance — usually 5–10% of the calculated weight — to arrive at the final quantity.
A quick cross-check: online calculators
Online steel weight calculators are handy for spot-checking individual bar sizes and lengths, but they don't replace a drawing-based takeoff.
References
- B.N. Dutta, Estimating and Costing in Civil Engineering
- Steven J. Peterson and Frank R. Dagostino, Estimating in Building Construction, 7th edition
- David Pratt, Fundamentals of Construction Estimating