Cutting length is not the same as the drawn length
The dimensions on a detail drawing are outside-to-outside; the bar you cut is measured along its centreline. Bend the bar and the steel on the outside of the curve stretches while the inside compresses, so the cut length is always shorter than the sum of the drawn legs. The correction is the bend deduction: about 2d for a 45° bend, 3d for a 90° bend and 4.5d for a 135° bend, where d is the bar diameter. Hooks are added the other way — a standard 135° hook on a stirrup adds roughly 10d per end, with a minimum around 75 mm. Get this wrong on a stirrup and the error repeats across every bar in the beam.
Weight: the number the supplier bills you for
Rebar weight per metre follows d²/162 for kilograms per metre with d in millimetres — a formula worth committing to memory. A 12 mm bar is 0.888 kg/m, a 16 mm bar is 1.58 kg/m, a 20 mm bar is 2.47 kg/m. In imperial, a #4 bar is 0.668 lb/ft and a #5 is 1.043 lb/ft. Multiply the cut length by the count by the unit weight, sum over the schedule, and you have the tonnage to order. Add 3–5% for the off-cuts you cannot reuse; suppliers deliver in stock lengths (typically 12 m), so the practical waste depends on how neatly your cut lengths tile into that stock length.
Lap and development length are code decisions
Development length (Ld) is how far a bar must be embedded to develop its full strength; lap length is how far two bars must overlap to hand the force between them. Both depend on bar diameter, steel grade, concrete grade, bond conditions and whether the bar is in tension or compression — and the three major codes (IS 456, ACI 318, Eurocode 2) reach different numbers from the same inputs. A common thumb rule of 50d for tension laps is a useful sanity check and a poor substitute for the calculation. Never lap all the bars in a section at the same place: stagger the laps, and keep them out of high-moment zones wherever the design allows.
Estimating steel before you have a schedule
At tender stage there is no bar schedule, but there is a concrete volume — and the two are linked by a steel ratio. Typical figures: 80–100 kg/m³ for slabs, 100–150 kg/m³ for beams, 150–220 kg/m³ for columns, 60–90 kg/m³ for footings. Expressed as a percentage of volume, 1% of steel by volume is about 78.5 kg/m³. These ratios are a sanity check, not a design: a heavily loaded transfer beam will blow past them and a lightly loaded raft will sit well below. Use them to spot a quote that is off by a factor, then replace them with a real schedule as soon as the drawings allow.
